The fastest time that you can level up in is 6 days and 20 hours (this time is even shorter for the first 3 levels and some of the later levels). To achieve this, (or something a bit more realistic like 7 days), you need to do your radical lessons and reviews for your current level as soon as they come in. Once those radicals are at guru level (should take about 3 and a half days at the fastest pace), you’ll unlock the next set of kanji for your level. At this point, you need to have done all the lessons for your kanji, so that they all have moved into your review queue. Now all you need to do is guru at least 90% of the kanji for your level which also takes around 3 and a half days at the fastest pace.
It helps to have a reorder script, so that you can do your radical and kanji lessons before your vocab lessons.
The key to this strategy is doing your reviews for the kanji and radicals as soon as they become available. You also have to be pretty accurate (especially on the radicals).