The fastest you can level up is about 7 days. That’s honestly a difficult pace to keep if you don’t have much free time so many people don’t go that fast. 18 is a bit long though for level ups but it really depends on what works for you. I recommend reading this guide to get an understanding of how things work and a solid example of a schedule.
My Journey of 368 days (+ The Ultimate Guide for WK 
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There’s also groups of people who take wanikani at a slower pace for their own reasons. How fast you should level up depends on your own situation and goals. Just make sure you don’t pressure yourself into going faster than you can handle, you’ll just burn yourself out.