DMZ2112
Chaotic Looseleaf
I think the important questions asked are:
1. How many sessions should it take to level?
2. How long should it take to reach level twenty if you play regularly?
What is "regularly?"
The more I think about it, the more I think that advancement rate is a useless discussion for the core rules. Every group is different. One might meet weekly for four hours a pop. Another might meet monthly for that long, while a third meets monthly but sets a whole day aside for 12 hours of marathon campaigning.
Both session length and session frequency are completely unknowable variables. Not only is it useless to talk about advancement in terms of real time, but it is also useless to talk about it in terms of sessions because even the difference between a four- and a six-hour session is going to dramatically skew expectations in a group that is advancement-focused.
The rules should specify XP awards and XP requirements and just leave the rest up to the dungeon masters. They're smart enough to figure out what works for their group, and they don't need to be made to feel inadequate because their gaming schedule doesn't match some Platonic ideal.