I hope I can make a 3.5-comparison comment without derailing the thread....
I was just noticing the same thing in my game, which currently isn't really planned to give the PCs downtime between days, let alone between adventures. This means that at 4-5 encounters per day, and 8-10 encounters per level, 3 weeks of in-game time is an entire tier, and two months is the whole game.
In 3E, the expectation was that there'd be about 14 encounters per level, and the reality at least in our group ended up that there'd be an average of 2 encounters a day — sometimes as many as 3, sometimes only 1. But that makes for a whole week in game to go up a level, and even with 20 levels instead of 30, it's about five months in-game for an intense campaign.
Either way it's ridiculous, of course. Since we only play twice a month, I don't really want to make it go slower; I suppose I'll introduce "and time passes" points eventually.