That sounds super frustrating to me. I feel like groups should be able to explore their new abilities for a while before leveling up again.A lot of '13th level' finishes here.
Levels 14-20 go past in a flash. Around 2 sessions each. A particularly challenging session can get you an entire level, and a session with a lot of talking increases the number by 1.
If you've gone that high, literally 14 more sessions get you to 20th.