Here is a very rough estimate. There are 9,352 total items on Wanikani per wkstats.
- 10 lessons/day: 935 days (2.6 years): finishing November 2028.
- 15 lessons/day: 623 days (1.7 years): finishing January 2028.
- 20 lessons/day: 468 days (1.3 years): finishing August 2027.
Keep in mind though that it’s not just about how many lessons you do each day but how often you do reviews, and the timing of your review sessions. You’re limited by how fast you can level up by the SRS timing. Leveling up is determined by guruing 90% of the Kanji, but before unlocking all the kanji you have to guru all of the radicals for the level. So leveling up quickly is based not only around how many lessons you do each day but how frequently and the timing of your reviews. If you want to learn more I’d recommend this guide: Wanikani is too slow and the Radicals are Useless. also Why Can't I just level up in one day instead of 7? - #51 by jprspereira
If you want a more chill plan while still having a decent speed, you could do 15 or 20 lessons a day and have a few review sessions a day that are spaced out well, so that you’re leveling up at a pace you’re comfortable with. Ultimately I’d recommend just doing what works best for you!