Before I learned to use reorder script, I used to do around 15 new lessons per sitting. It was around 2 weeks per level.
Time critical items are Radicals followed by Kanji (for most levels). You have to do all Radicals and Kanji (but not vocabularies) once the lessons appear.
The most popular reorder script is probably this one – Wanikani Reorder Ultimate [old version, not working]