I've only commuted down to 14F, mostly because that's as cold as it's been on work days since I started bike commuting. I use a new outfit for every 10 degree drop down to 25F; e.g., light tights to 35, medium tights to 25, heavy tights below that. Gloves are the hardest to get right.

But I'm not tough. When it snows, I want two tons of protection from the rest of the idiots driving around here.