I did level 1 for about 7 days then went to Level 2 for about 10 days. If I take more than 4 days off I go back to level 1 for a day or two then back up. I just recently felt like I could conquer level 3, but I'm not ready for it yet.
Personally, I think you should just progress to the next level as you feel ready for. Each level is a lot harder than the one before. Personally, level 2 is my favorite. I feel like a get a better overall workout on that level than the other 2.