IIRC, max speed was like 6 days 20 hours a level or something weird. Going slightly slower lets user stay on a much more regular schedule. I think it ends up being like 7 days 12 hours or something along those lines. For a long time I went with an 8 day schedule which was really to keep my lesson at the same time of day.
Really, just read the second post in this thread to get an idea of what to expect and what (not) to do or think about with regards to leveling. Fast or slow, there’s great advice in here.
It should also be noted that level 15ish is 95% JLPT N4 coverage (sort of) according to wkstats. But with the exam in less than two months…