It all depends on each individual person’s pace. For some people, 500 reviews in a week is quite normal and manageable. For some going even faster, 500 is actually low! But! Try not to compare yourself to others when it comes to your own personal pace!
For example, plenty of people would also consider 500 per week to be high, and there can be many possible reasons why that might be the case: Maybe they have a busy schedule and 500 is just too much to fit in with everything else; maybe they’re spending time learning Japanese in other ways (textbook, reading, etc.) and 500 is too much on top of that; maybe they are more learning Japanese as a hobby, and 500 just starts to feel like too much of a chore or a burden; maybe they simply feel too overwhelmed by so much stuff all at once. It doesn’t matter the reason, I’m just saying that everyone is different, and different people will have different natural paces.
Personally, for example, I have been ‘Durtling the scenic route’ for about a year now, because I realized that for me personally, ‘going fast’ was just causing me too much stress and I needed to slow down for the sake of my sanity! 
Now, I actually find pleasure and joy in taking my time with WK and enjoying the process as I go along. I no longer try to ‘rush’ myself faster than my natural pace. I just accept that my natural pace is going to be slower than other people, and that’s totally fine, because I don’t need to compare my pace to anyone else’s in the first place!
First of all, let me preface this by saying that the following advice is under the assumption that you are striving to find your natural pace – a pace that feels comfortable and sustainable for yourself. On the other hand, there are some (perhaps many!) people who are okay with going faster than ‘comfortable’, for whatever reasons. Maybe they want to get to a certain level within a certain time frame. Maybe they are cramming for a JLPT test, or actively/intensely studying Japanese. Some people just like to ‘speed run’ things as a fun challenge. The following advice is not directed to such people – and besides, there are already a lot of excellent advice for how to go ‘fast’ or ‘efficiently’, including several really good guides. But if your current pace is literally too fast for you, then the following advice may help regain control over your pace to bring it down to a more manageable, comfortable level. Okay!
First off: The absolute key to managing your pace is to adjust how many new lessons you do in a given period of time. If you are starting to feel overwhelmed with reviews, the solution (again, to achieving a more natural, comfortable pace) is not to ‘just do more reviews’! The solution is to slooooow right down your new lessons! Even slow it down to a stop for a while if you are absolutely overwhelmed with a mountain of reviews. In the worst case, you may actually be better off even resetting to an earlier level, essentially ‘undoing’ a bunch of lessons you took on that you weren’t really ready for.
So, if 500 reviews a week is too much for your natural pace (again, nobody else’s pace matters in this equation), then you almost certainly need to cut back on how many new lessons you are doing. While some people use the “0/0 challenge” idea to pace themselves – where you try to get yourself down to 0 Lessons and 0 Reviews at least once per level – and this can be good if you are trying to maintain ‘efficiency’, when you’re seeking a more natural pace, efficiency is not really what matters! What matters is sustainability!
Sustainability means: Not how many reviews can I handle on a good week, but how many reviews can I handle on a bad week.
Suppose in my life there are times when I just don’t have any time to do WK for a whole week at a time – e.g. maybe work projects/deadlines require all my attention periodically – which means my next week will have double the reviews (and therefore be considered a ‘bad week’). If I can only handle 300 reviews a week, then I should assume that my ‘bad week’ (which has double the reviews) will be 300 reviews. And thus, in order to safeguard against such bad weeks, I should strive to only do about 150 reviews (half of 300) per week for my ‘natural’ pace.
In that case, doing ~150 reviews per week on average, when a ‘bad week’ comes along, I’ll have about 300 reviews to do, but I’m able to handle that comfortably, and it’s no problem.
On the other hand, if I assumed that because I can handle 300 reviews a week, then I should strive to make that my average standard for every week, then when a bad week comes along, I will end up with a stack of 600 reviews to handle – and that might just be too much for me! This can lead to overwhelm and – worse – burnout. Burnout really really sucks; I highly recommend never putting yourself in a position where burnout will happen.
So, the first thing I would recommend would be to cut down on new lessons – and don’t worry about trying to achieve 0 Lessons each level, as that can ‘force’ you to do too many lessons, more than you can sustain, in too short a time.
So, depending on how overwhelmed you currently feel, you might cut down your lessons to about half of what you’re currently doing. But actually, personally, I would even recommend pausing all new lessons until your weekly # of reviews gets down to a level where you no longer feel any overwhelm. (Because overwhelm can lead to burnout, and burnout is by definition not sustainable.) But that’s your call as to how much to cut back on new lessons. Again, it’s about your pace, not anyone else’s.
Having reduced your new lessons down, your reviews will gradually start to ‘thin out’ until they reach a point where they are no longer overwhelming. (The great thing about this process is that overwhelm prevents enjoyment, so bringing your reviews down like this can actually bring back enjoyment to your WK learning experience!) If your reviews aren’t thinning out fast enough, reduce your lessons even more.
Eventually, you will be able to reduce your reviews to a sustainable pace – especially if you completely pause new lessons – because eventually you will do enough reviews that all your items advance to Enlightened and/or, finally, Burned. So, be heartened! Because this process is guaranteed to work! It just takes persistent daily reviews.
Which leads me to the second point: Do your best (がんばって！) to do at least some reviews every day. If there are times where you can’t do any WK for a week or whatever, that’s okay (and consider using Vacation Mode if that’s the case); when you are able to do WK, try to do it every day. Even if it’s just a single review that day, do your best to follow through on getting just that single review done. And give yourself recognition for even that ‘tiny’, ‘little’ success. Hey, it’s still a success, right?
The purpose of this practice is to develop a regular routine and habit of doing WK every day (when possible). The key here is to ensure that you succeed every day. Therefore, start your ‘bar’ for ‘success’ quite low! You can always raise it up over time, as any sense of overwhelm (or worse, burnout) begins to fade, and each ‘tiny’, ‘little’ success proves to you that you are making progress towards thinning out your review pile. Even a little bit of progress is still progress.
Another key point about ‘success’ here, is that – for the time being – you may need to limit how many reviews you do every day to the point that your review stack begins to grow rather than shrink! Again, this depends on your pacing and how many daily reviews you’re able to sustain, but if you’ve got too many items coming in (from too many previous lessons), you may just need to let them accumulate and not worry about them – for the time being. Remember, by reducing (or even pausing completely) your new lessons, you will be able to eventually get your Review stack down to 0 again. And that is the goal. But the only way to do it sustainably is to keep a steady, natural, sustainable pace. Eventually Apprentices will Guru, Gurus will Master, etc., until you will be doing more reviews per day than are incoming, and your pile will begin to shrink again. At that point, if you do not increase your # of lessons (yet!), your pile will begin to reduce faster and faster, until it eventually gets back to 0! Again, this is guaranteed to work, as long as you manage your # of lessons, and do at least some reviews every day.
Eventually, you’ll (finally) get to the point where you’ve got your reviews back under control. Getting them down to 0 at least once per level is still a decent goal – though again, getting lessons down to 0 once per level may not be a sustainable pace, that’s up to you.
At this point, you’ll want to start increasing # of new lessons back up to start filling up your review schedule once again. But this time, take it slower and more gradually. Again, it comes down to your own personal natural pace. For me, I do roughly 5 new lessons per day, but if I’m ‘short’ on reviews upcoming, I might do 10 a day. If my reviews are starting to pile up, I’ll skip a day or three of new lessons. I use my lessons as like a gas/brake pedal to maintain my pace at a comfortable, sustainable pace for me. Others do 10 per day like clockwork. Whatever works for them. Others do even more. Others do even less. All that matters is what works for you!
[Sorry for the super-long post!
TL;DR: Cut down your lessons (perhaps even to 0) until your reviews get under control. Then gradually increase lessons again, but not to the point that reviews ever become overwhelming again.]
Personally I would just power through without doing any new lessons until the workload is manageable.
