I bike until the temperature drops below-15 C (5 F). I don't know what the wind chill is on a moving bike but I guess it would be around -20 C (-4 F).

I have biked down to -20 C but with my commute taking about an hour in good weather (clear and relatively warm) too cold is not a lot of fun. We used to hit -20 C for a couple of weeks in February, but it's been different the last few years with a greater number of freeze/thaw cycles.