Won't know til I actually finish it.
It'll take until the to know whether I still have more sidequests I want to clear up, or if I want to start a new type of character & install some mods.
(My tendency with Oblivion is to play lots of different characters. I've made over a dozen new characters in that game, each with a different array of mods to try out. And I think I've only done the main quest like twice.)
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Now, the first time I played Fallout 3, I finished the Main Quest at around 25 hours in. But I played that character for another 50+ hours, because there were still things I wanted to do with it. So, even though I make lots of characters, there still good odds for me wanting to keep playing one of them.
Like I said, I won't know until I finish Skyrim's MQ the first time.
Edit: never understood the problem people had with FO3 - it was easy enough to keep playing, you just loaded the save you made before heading out with Sarah Lyons & Optimus Prime. No biggie at all. (And Broken Steel was terrible - I don't even bother installing that mess anymore. No-DLC FO3 is much more enjoyable.)
...regardless, the fact that FO3 had an end has no bearing on Skyrim. Different game series, different styles. :shrug: