The longest that I've been in a continuous campaign has been a few months. That's it. Probably fewer than 15 sessions. I've played in quite a few games. A lot of one shots.
I recently started DMing 4th. I ignore XP except as a rough guide. Very rough. Actually, I don't even really look at that anymore. They level twice every 3 sessions. A session of play, level halfway through the next, level at the end of the 3rd. They are now level 5 and will hit 6 at the end of the next session.
We play every 2 weeks if we're lucky. If not, once per month. As it is, it will take over a year of playing to go from 1-30. I've already told them that I don't know how long it will last, but I'm fine with that. Why? It's a new system. I want to see it all, and they want to see it all. Sure, we could sit there doing the same thing with the same abilities for 4 games in a row. Or they can master their tactics, then figure out the new changes that they need to make as new options are available. We've removed stagnation.
We just came from a crappy game where it was numerous sessions against ridiculously overpowered things to level. "We should get 1500 XP each for that encounter that took 2 hours and almost TPKed us... again." "You each get 300." Awesome. So, we left and decided that that wasn't for us. I hope that I can make this game last beyond my current ideas. I don't see them dropping out in the near future.
If you don't like the advancement by XP, then drop XP. It's not a resource that can be expended like in 3.5. As such, it's not necessary, except as a place-holder. It's a guide. If you don't like the guide, don't use it.
For the record: Our sessions are 6 hours, but we don't spend the whole time playing. We had 3 hours of general gaming discussion last time, 2 hours of plot development, and under an hour of combat that for a specific in-game purpose.