You don't really need to complete a run-through of Oblivion to have a blast with it and get warmed up for Skyrim. You can start it, do some quests, increase some skills and just run around in the world. I just tried some new techniques along the lines of a semi-Pacifist character to see how it would go. I've gotten through a few quests with zero kills...the Azani Blackheart with Modryn Oreyn and one of The Collector Ayleid ruin dungeon crawl ones. It was fun and I learned some new things.
You can get through a FNV in 150 hours or less depending on how much you want to do...way less if you don't do everything. Then you can do more next time.
I just went back to a FO3 run I had going and it's fun. I keep a few games going so I can jump to one if I feel like it. I said Oblivion for you by the way.

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