I like how everyone is saying : Hurr durr, Fallout 3 / New Vegas does it good.
No guys, it doesn't. You get far too few Exp to make it worthwile, it is an added bonus, but you won't get a satisfactory amount of it. It , of course, is good that they give you exp at all, but it is way too small to matter (matter later on).
Onward to the actual question and more of the arguments:
Oblivion... now that was interesting (same goes for Morrowind). It's reasonable enough, but there has to be certain endlevel, because otherwise (as someone stated) you will be a master in everything.
Planescape Torment: Well duh, the game was more like a book so it was reasonable. Also it was heavily story-centered and wanted the player to be confronted with real issues and award long discussions. I.E. the brothel, which was a wonderful way to get more experience, as player and as nameless one.
Deus Ex / VtM:B : Now those were really good, especially VtM. You got your experience NEVER from killing anything, but rather by completing objectives and sideobjectives. Minor spoiler : you could either kill someone, take the key and get into a certain room (giving you 1 EXP [different system, raising a skill costs 3-20 exp] or you could persuade him (giving you 2 for persuading and 1 for getting into the room).
Now obviously i tend towards the last system, which gives a smooth talker as much, sometimes even more freedom than the brute that's out for blood. So yeah, that would be awesome, in most RPGs however the game is balanced around killing, so you would have to take a completely different design from the very beginning. VtM:B had (off the top of my head) about 30-60 enemies to kill, if you were bloodthirsty (and didn't kill inncoent people). A regular RPG gives you that many to kill to reach lvl 2 or 3.
Also your example for granting EXP depending on the length of the session: That's the job of the DM in a pen&paper rpg, they also give you exp for doing something funny, helping your mates, playing your character. etc. That would be the best way of giving exp, having someone evaluate your playstyle etc.. But we have to stay within the possibilities of the game, so yeah. long rant over :<