I think it's a balance between the attention span/availability of your players and the length of your story arc. Strike whatever balance you need for your games.
I am fortunate that I've been with the same core group for something like 10 years, more for some of the players, but I gather that this is not the "norm."
For a lot of groups, even finding time to game weekly over the course of a year would be hard, and that's what, 50 sessions, if you're lucky, accounting for the odd cancellation. In fact, it's probably optimistic. So, you go 50 sessions, times three encounters per session, and you're at 150 over the course of a year. 50 encounters to the level puts you at... level 3? After a year.
Eh, it's a little slow for my liking, but if that works for your group... knock yourself out.
Then again, if my memory of those older editions serves, what constituted an 'encounter' took a lot less time to resolve. So, yeah, I guess if you wanted to plot it on a graph, one axis could be 'time available' and the other one could be story progress.
The other solution is, have players make new characters often and you can still get lots of mileage out of those low-level dungeons. Or repopulate some them with higher-level critters.
The main thing is that everyone is having fun.