Certainly down to -10 F, canister temperature. But before giving up all hope beyond that, stop and think for a bit about how you carry and use your stove.
First of all, let's assume you are carrying your stove and canister in your pack against your body. Now I find I can carry 1.25 L PET bottles of water that way and they never freeze. So anything placed near my back in my pack is unlikely to drop below 0 C. For butane/propane mix in an inverted canister, that's warm!
Second, I usually have water in my pack - water, not ice. So if I place the canister in a bowl of water during use, or fill the inverted base up with water, that is going to help keep the fuel around 0 C. For butane/propane mix in an inverted canister, that's warm!
Third, once I have the stove running I can always arrange that the canister gets a LITTLE radiant heat from the stove - enough to keep it around 0 C. (NEVER let it get too hot to touch comfortably.)
So just how cold does it have to be before you can't use an inverted canister stove any more? Very, very cold if you are smart.