When it's below -15 C, I don't ride. If it's not (and there's no blizzard), I ride. The limiting factor is that I don't wear/have any special gear - just skiing jacket and gloves, and below -15 C my face and thighs start to freeze (on >1h rides the feet get cold too). I have ridden a couple of times at -20 C, but that wasn't an enjoyable experience without dedicated gear. The biggest concern usually is stretches of unmaintained roads - bumpy, uneven, rutted ice which may be covered by snow or slush.