there's a EGG article in The Strategic Review (go hunt it down in the Dragon Mag Archive) where he talks about how slow XP gain should be. His preference was that it be very slow indeed, much like Diaglo's example.
As for some folks anecdotal evidence of how long their games lasted beyond WotC's survey, I think they're missing the point. Assuming WotC's survey was fairly accurate, which most folks assume it was, then the AVERAGE gamer's experience was 12-20 sessions before the campaign folded up. All of this was pre-3.x, so we're talking 1e and 2e players here (and Diaglo).
Fact is, for most gamers, campaigns don't last, and they usually die before the story is finished. This is due to lots of silly people problems, like scheduling, crappy DMs, and sociopathic players. Now there are bound to be players who had longer campaigns, odds are good, those people are what pushed the average campaign duration UP to 20 sessions.
Now I assume that when WotC saw that campaigns maybe last 20 sessions, and tended to end around 12th level, they decided to try to speed up advancement, so players could at least experience higher levels. That's nice for the players. And I see that some of you say campaigns end early because it's too hard for DMs to run. I suspect that while that is one challenge of high level campaigns, that premature campaign termination is more frequently caused by other factors.
What kind, lessee here, what's been rehashed before on EnWorld about why campaigns die:
players keep missing events
DM isn't prepared
It's hard to DM high level games (throwing a bone for some of you)
interpersonal problems between players (arguments stop the fun)
Crappy DM, so players exit the game (I've bailed on crappy DMs and watched the campaign fizzle)
Lack of interest from players
If each one of those items has a 1% chance of happening and killing the campaign PER session, how many sessions would you have before the campaign ends. I'd guess that you'd be lucky to get 20 sessions in.
I've been a player or DM in the following sessions:
F/T: about a year
M: 4 months
M/TM: 5 years active (every 2 weeks)
T: 6 months
B: 12 months
R: 12 months
C: 12 months
Brb: 14 months
I've also probably played in 3-4 campaigns that started and fizzled by the second session (ie. were intended to be campaigns, but flopped so badly, they never got a second session).
I've played 4-5 campaigns that maybe lasted four sessions, also flopped due to busy schedules and lack of interest.
DMing campaigns
8 months (scheduling)
3 sessions (fizzled)
6 months (interpersonal)
18 sessions (over a year and a half real time and still running)
I suspect there are more people with experiences like mine.