I'm surprised how many people I've heard of that have just made on character and done everything with them.
My first playthrough, I was a redguard weaponsmaster who believed in justice (yes, I roleplay when I play). I completed the main quest and the Companions quests, and any side quest that involved killing someone "evil".
Second playthrough I was an evil altmer mage. Completed main quests, Winterhold and Imperial quests (in retrospect, the Imperials seem a lot better than Stormcloaks - really I should have done their quests with this guy). I did any magic based side quests (i.e. Daedra),
Third playthrough was a khajiit thief who later became an assassin. He did something if he felt that there was a profit in it. With him I completed the main quests, Thieves Guild, and Dark Brotherhood, as well as any treasure hunting side quests. He was also my only character to reach above level 50.
All in all, pre-Dawnguard, I probably but about 200 - 250 hours into the game, and it took me to around January time to do everything.