As for my own attempt of this, I used a male khajiit thief born under the sign of the thief. I used my majors often to ensure I'd level up a couple times (getting Umbra at level 1 would take too much time), and I did complete the Siege of Kvatch (and the cuirass lasted me the rest of the playthrough, fitting for the Hero of Kvatch) and saved Jeelius. I finished it on Heartfire 1, 10:39 AM. The file says 6 days, but since it says 1 day has passed when you start the game, the oblivion crisis only lasted 5 days, 9 hours and 39 minutes. I know there's a number of things I could have done to go faster (In particular, I had to waste a lot of time waiting for the map to the dagon shrine to show up, and could have stolen a black horse at the start), but it's still an impressive display having the disaster last less than a week. So, anyone else want to give it a try?