Oblivion is such a massively huge game (especially when you consider the downloadable content including the huge Shivering Isles expansion pack) that even if you played it 30 hours in a row, it would still feel like you were just popping in for a bit and popping out. There is so much to do, and so much freedom in what you do when, that it can easily be enjoyed in short bursts from time to time, set aside, and returned to much later. It's certainly not a game where you feel like you can finish it now
if you just power through for a little bit longer.
The real issue is this: it is entirely possible to play in short bursts, but highly improbable that one would be able to do so!
It sucks you in so well that it is very tough to put down once you get going in a session.