My zone of comfort expanded pretty quickly during my first winter of commuting. Seemed like every week from Nov to Jan there was something different to contend with--ice, more extreme temps, wet snow, slush, etc, different types of snow.

My limit is more than 3-4 cm of fresh fallen snow. So if it's fallen during the over night, then the morning commute is a no go. If it falls during the day and the plows and salters have had a chance to come through, it's good to go.

Temperature wise I haven't really found my limit. I've done it down to -20*C.