I think I'm stating the obvious, but it seems important to me to state!
To me, the main question on whether a given amount of time is sufficient, whether 2 hours or 2 days, relates directly to the purpose of the gathering. For example, looking at the future of an organization in 2 hours and expecting clarity to emerge out the other end is silly. On the other hand, having a great conversation about xyz with no expectation beyond that (although great things could happen) is entirely possible.
