The first tournament in our winter series the temperature was 0F and we still got 63 people to show up
But I have to say, when it's colder than that I probably won't be throwing many practice rounds, but I'll still play a tournament if scheduled. Last winter I think I played one in -15F.
The only time I left the course without playing a full round was when it was very windy and snowing those small ice crystals that hurt your eyes when you face the wind. I couldn't keep my eyes open at all when facing the wind. That's when I gave up and went home. But it was only a practice round. Had it been competition, I might have toughed out even that