Your level up time is going to depend on several factors as well as on your approach: the number of lessons / day, the number of radical and kanji in a level, accuracy during reviews of radicals and kanji, timing of reviews, whether you use any scripts.
I do 11 items a day - I am using the WaniKani Lesson Filter, but that is actually to spread out my radical and kanji lessons (I only do 3/day w 8 vocab), not to do them immediately they are available. I have a 96%+ accuracy rating. I finished level 12 (14 rads, 39 kanji) in 19 days.