My number used to be 50 degrees. I wouldn't play when the temperature went below that. Now, with a few kids and opportunities to play getting harder to come by, my number is closer to 40. I definitely don't play in the 30s. Just not worth it. But, with the Under Armour-type shirts you can be pretty comfortable in the 40s.
I found myself putting through a light layer of snow once and had to call it. That morning started over 50 and the temperature plummeted throughout the round.
What's your number?