Thanks, all. The consensus does seems to be that the combo of "no communication and no show" is the dealbreaker and that 15-30 minutes is a fair "grace period" in such situations.
I did find that communicating the boundary of my patience ("if I don't hear back by ___") proved a sanity-saver for me yesterday, so I'll likely carry that forward.
And for those who might be interested:
The gent did ultimately contact me, about two hours after we were set to meet. His story -- if true -- would suggest that he simply had a very bad day, complicated by some truly bad luck. So I'm willing to accept that he may actually be legit, given the enthusiasm of other posters. As for me, I go back and forth on whether I believe the gent's story as I also take come comfort from the not-insubstantial pile of dollars that remain in my pocket.
RyanDean's post in grace period was marked as the answer