Leveling depends on guru’ing all of the radicals for your current level + 90% of the current level’s kanji, thus leveling quicker depends on how soon you’re doing the radical and kanji lessons to get them into your review queue and then getting those reviews correct 4 times in a row at the SRS intervals (4hr/8hr/23 hr/47hr) in order to guru them
The 7 days/level you’re seeing comes from doing the radical and kanji lessons as soon as they’re available and then doing the corresponding reviews as soon as those are available. Since you unlock two batches of kanji lessons/level, going max speed would allow you to finish a level in 6d 20h (and for the “fast levels” where you only have one batch of radicals/kanji necessary for leveling up [levels 26, 46, 47, 49-60] you can level up in 3d 10h at max speed)
Here’s an excellent explanation on how to develop an efficient WK routine ^^