It less depends on the temperature and more depends what the river looks like, if it's slushy I stay away; if it's clear I'm going fishing if I can. Still have to be careful for those ice shelves though...
1. At the end of every cast get in the habit of taking a glance upstream to see what's coming.
2. Always know what the general depth/speed of water underneath the ice you're standing is and if it's safe. If you're ever unsure, move on.
If it's hovering around -5 to +2, your guides are going to ice up a bit, pretty easy fix, you'll get good at clearing your guides quickly after doing it a couple times.
If it's in -10 land you're only going to get about ten casts before you'll have to clear your guides...But that's the least of your worries. The frustration of your flies turning into little frozen fly line dingle berries and getting contact frozen to everything that isn't a fish is what keeps me off the river at these temps
And at -15 (yup) you go to the river, take one cast, and go home.
tl;dr Fly tying exists for a reason.