For me going in the fast lane was ok until level 20 perhaps. I had my sentence mining routine and my graded readers to put those kanji and vocab into use ASAP, so that was holding all in place (you could say that was “my secret”, after all I only really feel I learn anything when actual context strikes, I think of WK more as kanji awareness tool 
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By the time I reached level 25 my accuracy for reviews drastically decreased, as well as my motivation for learning so many kanji in such short time, mainly because I was able to read and understand much more; I was eager to read more, watch more shows, etc… But eventually that became incompatible with a new level each 7-8 days. Some of the new kanjis became less used in my current readings, specially once I decided reading native material (children stories), as those books replaced them with hiragana.
Eventually I decided on slowing down, do less lessons per day on WK, and focus more on immersion and the vocab I was learning there.
Now I’m on a healthy 11-12 days per level average, and I think it’s much more sustainable in the long run. In any case, I’ve realized that a fixed number of days it’s not something important. My main focus now is immersion and doing daily reading on a consistent basis. It’s more fun and actually I think I get the most of anything I learn here, as WK is now a tool for getting me into shape for more advanced readings 
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