I seem to be one of the sailors here with a lower acceptable range (no gusts over 20 solo). But if I were asked what is my cut off, I really couldn't answer it. I think we'd all want to stand on the shore and watch the conditions for a few minutes before making a call. I think back on days that I have called it and remember how long I anguished on the shore before deciding it was too much wind. I mostly cruise around though and I would probably feel added pressure if it were a race.
I also remember a day that there was too much wind and my brother-in-law was in town for a sail. He was willing and it was an onshore breeze so we rigged up and took off. It was an absolute beating. We stayed out for about 10-15 minutes and easily agreed to head back in when the jib tack blew out. I have to admit that the broad reach back to shore was AWESOME! We popped off a deck port bouncing over a wave but no damage. I won't confess to what numbers I had in my head, but when I got home and check SailFlow.com, it was only 18 knots gusting to 23 knots.
I guess the short of what I'm trying to say is that there are 15 knot days that are incredible sailing, and there are 15 knot days that I won't even consider going out in.

-Rob V.