I just tracked the amount of time I spend on WK daily, and I don’t know how helpful my experience is for you (since everyone is different), but I do spend only a little more than 30 minutes (total) a day on WK alone, and I am progressing through it at a rate of about one level every 14-16 days, so I think it’s definitely doable!
When I tried tracking my time a few days ago, I found that I spent a total of 22 minutes on 116 reviews, across four sessions (only three were what I’d consider essential, but I did a batch of reviews early because they were available and I had the time). In addition to that, I spent 9 minutes doing 13 lessons that day.
However, I also spent 5 minutes drilling myself on the new items with the self study quiz script immediately after my lessons (which helps my retention a lot, therefore reducing the overall review workload), and 2 minutes drilling myself with the leech training script.
Grand total: 38 minutes a day strictly on WaniKani, with a rate that should roughly let me reach 25ish levels in one year. I haven’t quite reached my full workload yet where burns are coming in regularly (the first three and a half levels were slow for me), so the amount of time I’m spending daily will be going up a little bit, but I don’t think an additional 10-13 reviews a day will add much more than a few minutes.
Of course, all of this depends on how fast you are at doing reviews, and how good your accuracy is! Less accuracy = more reviews coming back. If you’re better at Japanese than I am, you might be able to do more reviews in less time.
For what it's worth, these are my stats:
Another important caveat: I am also doing Kaniwani at the same time, so that strengthens my memory of the WK items, and I am also learning to write kanji, as well as using the textbook Minna no Nihongo to learn grammar.
So, on top of those 38 minutes on WK, I also spent 22 minutes doing 129 reviews on Kaniwani, then 13 minutes writing kanji, 6 minutes doing Anki reviews (for MNN vocab), then finally 1 hour and 12 minutes on MNN itself.
All in all, it added up to 2 hours, 31 minutes of Japanese studying that day. That’s a little on the heavier side for me, at least with the amount of textbook study, but I think I regularly spend one and a half to two hours studying Japanese each day, plus a fair amount of passive immersion. Some of what I do is perhaps “optional” (many people don’t learn to write kanji, for example, and other people only practice recognition and don’t bother with recall, and some folks don’t need the self study quiz or the leech trainer), but it does all tie into my WK progress because regularly using the kanji and seeing them in other contexts helps me remember them.
If your time/energy is very limited, I think it would be fairly doable to go at about half of my WK pace, and do maybe 6-8 lessons a day (I do 10-13). Personally, I like to spread out the kanji throughout the level (I do 3 kanji alongside vocab 10 lessons every day until I run out of kanji lessons, then just 10 vocab lessons each day until I guru the last of the kanji and level up). I keep my daily workload aggressively consistent so that the time I spend on WK/KW doesn’t vary a lot day to day, and so far, doing a set amount of lessons each day and never wavering from that is a great way to keep it consistent.
I think 100 or even 80 is way too high of an apprentice count to aim for, though. Mine is currently at 64, and that’s enough for me! I would also not recommend leaving reviews hanging if you’re unable to complete all of them each day. If you find yourself running out of time to do them all, it’s better to decrease the amount of lessons you’re doing to reduce the workload instead.
And of course, you should try to make sure that you leave enough room in your schedule for grammar study and (at some point) immersion. My recommendation would be to build your WK schedule so that WK takes up less than half of the time you have available to study Japanese. The WK workload will steadily increase until you get to a point where your burn reviews are coming in, so you’ll want to start off spending less time on it than you technically are able to. If you get a very consistent schedule established, it should be easy to keep up the habit of doing it every day (and also add in blocks of grammar study/immersion) because you’ll be able to plan around it and know how much time you’ll expect to spend every day.