My 4e Loudwater campaign is an epic with dozens of adventures, started in 2011 and we just played the 81st session - http://frloudwater.blogspot.co.uk/ - the 4e PCs are 22nd level but the actual campaign scope feels more like ca 12th level in AD&D. However many of my campaigns recently have been more like 20 sessions, with a limited scope. My Pathfinder Curse of the Crimson Throne campaign http://smonscurseofthecrimsonthrone.blogspot.co.uk/ will only be 5 full adventures (having skipped the Book 4 railroad) over something like 35 sessions - and it could stand to have been shorter, Paizo stick in a lot of padding to get the level-ups they want. These 20-35 session campaigns typically run around 18 months, with a defined scope.
I think D&D design needs to accommodate both sorts of campaign. I'm in the early stages of planning for my first 5e campaign, and it looks as if the system's XP track, bounded accuracy and lack of magic item reliance should be well suited to lowish-magic exploratory sandboxing (thinking about using it for Caverns of Thracia), or possibly for some of the lower-powered Paizo APs (thinking about using it for converted Skull & Shackles, a piratical AP whose naval theme makes little sense in Pathfinder rules where magic does everything).
Read more: http://www.enworld.org/forum/showthread.php?428536-Gaining-Experience/page6#ixzz3UA9DXmHl