Right now my mod list includes The Lost Spires, where I started picking away at the (non-respawning) first dungeon at level 2 and have just finished it off and got the plot started at level 7; there's Vilja, whose first quest requires an ingredient that won't become available until by any means but the console about level 10 or 13; I started Frostcrag Reborn at level 2 and had to quit because any one of the several free weapons there is worth more money than my character had ever seen total; another companion I'm considering has a major quest each for levels 10, 20, 30, and 40; and even Oblivion itself has daedric shrine quests unlocked at level 20, which are the only things in the whole vanilla game I've never done.
So apparently people hit level 10 by the sewers, start all their quests at level 14, and finish with their characters at 45. I'm probably 30 hours into my level 7 character, and I've only hit level 20 once in five years of this game. I run out of stuff to do around level 17 usually. Am I doing something wrong? Is your character really high level and still going? If so, do you use some kind of overhaul, or some trick to level quickly without the game getting annoying because your skills are too low for your level? Or do you just spend lots of time in questless dungeons, so that by level 20 you aren't nearly out of quests to do yet? How long do characters last for you?