Our longest alpha ever was about a week, I think. Maybe two weeks?
Anyhow, we don't tend to do very long alphas. In terms of number of testers, I'm not yet sure. And not sure when exactly alpha will start; I've had some things to take care of this morning that have slowed me up (again). If alpha winds up starting tomorrow night, then beta wouldn't start until sometime mid or late next week. And that's looking likely anyhow, for other reasons.