bbPress.org

The shorter the better IMO. How about six months?

That should be enough time for people to sort out their plugins. _CK_ posted somewhere that their plugins will be updated in November (I think) and since a huge proportion of the most popular plugins are by them I’d be inclined to base the date on that.

You don’t want to end up in a similar rut to how WordPress ended up supporting the 2.0 branch for a stupidly long period of time despite nearly no one actually using it.