theres no set time for how quickly you level up but you generally level up quicker by doing quests and furthering the storyline.
Its a slower process if you just kill things and ramble about, I know it sounds stupid but have you tried sleeping in a safe place, when you do that you can choose what attributes to level up once you hit the EXP points required for the next level.
brilliant game, HHHHHHHHHHHUUUUUUUUGGGGGGGEEEEEEEE though, you never truelly finish it, too many things to do and quests that just pop out of nowhere.
There doing a game of the year edition at christmas I think, it will have the two PC expansion packs with it as well, worth buying for them I think.