I'd say about four months of slow grinding I'd get to the end of it, but with my other three good friends, not alone. Solo would probably take about half a year (for experienced players).
All rumors considered, I don't expect Inferno to be only for the best of the best, it'll probably be suitable for 1/5 of the community, especially wow players somehow. I don't know but I just feel it they'll do something to let them get to Inferno.
Somewhere along these lines! I speak metaphors a lot.
The Freaking Release!!