When GMing a PbP game, how should one decide when a "game session" has passed and the PCs' once-per-session talents and powers are reset?

The rulebooks suggest that a session should last for a few "major encounters."

To approach the question differently, if you already have a good handle on the length thing, declare the end of the "session" whenever you would normally award XP. For me, I knew it when I saw it. It was pretty obvious when it was a good time to stop for XP and to reset talents and strain.

My advice on the matter: be generous with the XP whenever you do give it out. PbPs can move so slowly that players get disinterested. A more rapid progression of XP can help counterbalance this tendency.