Hopefully Bliz manages to make Inferno as challenging as they claim it to be. If so, I'm not gonna be the first to finish act 1/2 and it'll take me 'bout a month to finish up to act 3. As act 4 shall be the most challenging thing they could ever invent, it'll be unlikely someone will beat it as fast as "wow hard modes" since those really aren't hard (anymore).
Before the announcement that they changed inferno from having a flat difficulty to progressively harder I would have voted less then 1 month. With the change I'm thinking/hoping it will take more then 3 months to finish act 4 since now they can tune act 3-4 to near perfect inferno gear instead of making inferno doable with hell gear.
1 to 2 months. I might get a character to 30 and realize that i don't want to continue with them for the moment and try another. I have nothing going on for a month or so after release so i expect to be playing this a lot. For inferno alone, i hope it take a good long while, i like having to farm gear in lower level content to be able to progress to the next tier of difficulty. Also i don't want finish inferno too fast and get bored with the game.