Over 400 hours on Oblivion, 150+ hours on Fallout 3.
When I played Oblivion, I didn't have the heart to start over either, but that's because I got totally sick of the underlying gameplay by the time I had done perhaps 10-25% of it at most, and suffered through the "hurry up and save the world right now" MQ.
In MW, I still haven't played one of the three Great House quest lines at all (and haven't finished another of the three, but I'm currently working on it), haven't done at least 3 of the other faction questlines, none of the Vampire or Werewolf quests, didn't complete Bloodmoon's MQ, and still have tons of sidequests to try, either because they didn't make sense for my previous characters, or I just didn't have time for them before those characters became overpowered (after well in excess of 1000 hours of playing the game over several years). I figure it's better to do them sometime with another character than stroll leisurely through them without a challenge after I've already beaten the toughest adversaries in the game. I don't feel a need to rush out and "do everything" with a single character, and the game certainly doesn't force it on you. I consider a "good video game" like a fine wine: to be sipped and enjoyed, not "chugged" for a "quick buzz".