My limit really depends on how far I am away from security. Wind chill plays a major factor as well. Nothing worse than fighting your way through the wind, working up a sweat, only to have all your heat blown away. Cold sweats, me no like. MTB in the forest helps with this.
I've flatted 40 km from home, 2.5 hours into a ride on the road. Temp was -8, -15 with the wind chill. Miserable, miserable.
In general, I won't venture much more than 10 km from home/car whether on the road or MTB when the temperature is below 0. This essentially means no road riding below 0 anymore.
I'll MTB to -5 to -12 and venture up to 5-10 km from home (1-1.5 hrs total).
I'll go down to -12 to -15 at a place like Punslinch where I am never far away from the car.
Below -15, I play some extra squash.
Commuting is another story. 11km one way (hence, my tolerance of 10 km). My limit is -25 with the wind chill. But that tolerance goes down, as the wind chill component goes up. I'm more likely to ride on a day that is -25 and calm, than -5 with a 50 km head wind in the morning. One of the few times I've flatted during my commute was the coldest morning last winter (or the past few winters) in Hamilton at -24. Blessed was the bus system that morning (and why I ride along their routes)