Try an hour, I'm fat'n'out of shape myself and that's a basic rule for me, although I'm often riding more than an hour, an hour's a good guideline. A nice ride for me is here to Palo Alto, a few towns over, which takes an hour, then have some coffee etc in downtown Palo Alto then take the train back. (I take my bike on the train, the ticket's cheap, then bike from the train station home, which is a short hop.)