You don't want to know!
But...
You asked for it.
[HEADING=2]4,000 hours. FOUR. THOUSAND.[/HEADING]
In Oblivion. I've done it all, played every playstyle, memorized the map, experienced every glitch, and generally mastered the base game. All on the 360 version, no less. Have the PC version and a decent laptop to run this now-old game, don't even want to risk my soul by dipping into the mods.
About 100 in Fallout 3. Didn't like it as much.