How long does it take to finish WaniKani?

That depends on you.

If you do your reviews on time and get most of them correct, it’s possible to get through all 60 levels in just over a year, or 368 days for one WaniKani user. That’s breakneck speed, however.

Getting to level 60 in a year and a half or two years is a much more reasonable speed.

That said, go at your own speed. You can always reduce the number of lessons you do to slow down. Or, do them when they become available to go at top speed. Learning ~2,000 kanji is going to be a long path regardless, so go at the speed that will keep you in it all the way to the end. Compared to traditional methods, it’s going to be faster no matter what.