It depends on how long the ride is. I'll do anything down to -5 C (that's high 20s in American I think.) From -5 to -20 I'll ride for up to an hour, but not much longer. Below -20 I just don't have the clothes for it yet. I know guys who were still riding when it was -50 this winter, and they were wearing shorts + light tights + heavy tights. I'll get some stuff at the end of season sales this year and be ready to go through the winter next year.