This has already been covered by other people, but in case you are wondering WHY they are telling you it is possible but quite challenging, let me give you some numbers. Assuming you have read the introductory guide (and if you haven’t, I highly recommend it), then you will already know that WaniKani is built on SRS principles, aka Spaced Repetition. This means that you will be given new lessons to learn, then asked to review these lessons at spaced intervals. If you successfully review the material, it will move up an SRS level and take longer to review. Correctly review the same item enough times at it will “level up” from Apprentice to Guru (and beyond). Level up 90% the radicals in your current level and you will unlock new kanji (and probably some vocabulary). Level up 90% of the kanji in the current level and you will unlock radicals for the next level (and also a bunch more vocabulary). You do not have to level-up the vocabulary words to advance to the next level, but you will probably want to learn them, since they help with memorizing kanji readings.
Now for the important bit. WaniKani’s SRS levels break down like this …
2 days (Guru)
4 months (Burnt)
If you add up the time it takes to reach Guru, it comes out to roughly 3.5 days. So that means if you do all your radical lessons at one time, then hit your review times perfectly, reviewing as soon as they come available, you could unlock the kanji in less than four days. If you then do your new kanji lessons immediately and keep hitting all your review times perfectly with minimal errors, you will be able to complete a WaniKani level in about one week (actually a little less if your timing is flawless). This is the FASTEST possible speed and would involve waking up at odd hours sometimes to do your four or eight hour reviews on time.
If you are completing a level every week, then that means you are finishing four levels each month or 48 levels each year. To reach level 50 (the original “end” of WaniKani), it would take just over one year at MAX speed. To get to level 60 would take another couple of months or less, since the higher levels don’t have many radicals.
If you have the free time and dedication to go full throttle, it can be done. But it is not the “average” or even the recommended rate. It takes a lot of effort to achieve those results. It’s not going to be worth it for most users. A more realistic competition time would push that timeline out a little further. For example, if you are completing a level every two weeks, it will take you roughly two years to reach level 50. This gives you twice as much time to finish and gives more cushion for life intruding on your WaniKani schedule.
Keep in mind that you do not need to reach lvl 50 to start seeing results from your WaniKani practice. You can start using the kanji you learn right away in your Japanese studies. Ideally, you should be taking time to learn grammar and practicing your reading/speaking skills through other means, so that when it comes time to call upon your kanji knowledge, you’ll know what to do with it. If your end goal is to move to Japan in one year, then you can certainly aim for that. But you should not expect to arrive in Japan as a fluent Japanese speaker. A year is not that much time, in the context of language learning. But you can accomplish great things in that amount of time, if you are willing to put in the effort. Getting started with kanji learning is a great first step. I would also recommend diversifying your efforts. You will need a solid foundation of Japanese grammar to understand native material and a LOT of practice reading and listening to Japanese.