30 days and 9 hours (level 23)
Went to Japan for two weeks, during which I did zero WK. I thought I had put on vacation mode, but apparently not, because I came back to a ton of reviews, and it took me quite a while to get back on track, my next three levels was 12, 17 and 15 days.
You can check some older responses here (I found them while looking for my charts that I posted a while back):
https://community.wanikani.com/t/what-was-your-slowest-level/23048
EDIT: Stopped being lazy and just “made” a new graph:
