In Montana, just like many places out west, or the upper midwest, or for sure Canada, eh? Alaska too....it's pretty much a given for hunting in sub-zero. Lot's of mornings I've spent in 20 below. I can take that for a time. Our problem around my house is the cold with a killer wind, that can drop windchills easily to minus 60 or 70. Having said all that, I never really keep track, as it is just a normal part of hunting here, but here's a few pics I've pulled up from photobucket of some sub-zero hunts here.
Last year, temps were well below zero, but a 40 to 50 mph wind made conditions unbelievable cold! I figured I could last perhaps an hour....so went out (cause you can't kill nothing at home and the season was running out) and took this doe. My first harvest with a recurve in 25 years.
I took this cow elk several years ago, with a board bow I was sure would blow on me. Temps were easily near the 20 below mark, but no wind, so a 3-mile hike, spot, and ambush ensued...with good results.
And here's a few more sub-zero hunts with some recognizable characters, and another last day buck.