Generally, operation down to 20F/-7C over the life of an isobutane-propane (no n-butane) canister is a reasonable expectation. As has been noted, the canister temperature will typically fall below the air temperature, so you need to take some steps to keep the canister temperature up. Generally putting the canister in (liquid) water is pretty safe and will keep the canister going. If things get really cold, it can be hard to keep that water liquid.
However, as Bob mentioned, the higher you go, the lower a temperature you can operate a canister stove at. The following graphic may be useful. Sorry it's in English units only. :(
P.S. Remember that you need about 10F/5C degrees "clearance" above the vaporization (boiling) point in order to have enough pressure to operate a stove. For example, at sea level, isobutane vaporizes (boils) at 11F/-12C, but don't think you can operate a stove that low. You need some "clearance" where the fuel temperature is above the vaporization point. At sea level, about 20F/-7C is about right. At 10,000 feet, the vaporization point is going to drop to around 2F/-17C -- but you still need some "clearance" so more realistically at 10,000' you can operate your stove as long as you keep the fuel temperature at about 12F/-12C (pardon my rounding errors).