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.
[emphasis mine]
All this a hundred times!
Tell me how much XP defeating (note this does not necessarily mean slaying) an Ogre. Tell me how much/many gold (or, I suppose now would be silver) pieces = 1 XP. Give me guidelines/suggestions
for the DM to calculate "completing a mission" or "story arc", "successfully negotiating the trade agreement" or "resolving the inter-kingdom conflict", "saving the princess", "exceptional role-playing/advancing the story", etc...
One can play a four or six hour session one week with minimal killing/treasure accumulation/accomplish little. A two hour session a month later might include a massive combat with the BBEG! The DM is supposed to say, well, we played two sessions, everybody have a cookie!
Absolutely not. Not on my lawn!
The only thing saying "level at X many sessions", placed in the books even mentioned as a guideline/option, does is instill and encourage player entitlement. The only thing saying "kill 20-30 goblins to get to level 2" does is instill and encourage murdering psychopaths. Neither of these are games I want to be in. No, no, and no.
For those who don't use XP in their games...or prefer a "level up by session" mentality/playstyle, its moot. They still won't use XP so it doesn't matter what the books say is an appropriate amount.