In AZ, all year, no problem. My commute ranged from 14 to slightly over 20 miles each way.
In KS, I was lucky to be able to commute 6-8 months, depending on how early the first ice storm hit. I hate the cold, and the ice and ruts in the streets would last for months, since they didn't use salt at all.
In Seattle, 12 months, but there was usually a 2-3 week period in the winter where it just wasn't feasible, and again, I hate the cold, and wasn't willing to risk putting studded tires on my bike just for that few weeks.
Southern OR was about the same as Seattle. I was about to pull the trigger on a trike, since it wasn't terribly cold, but was cold enough that random bits of ice would cover the trail. (Then I moved back to AZ....)
Now that I'm in UT, I have my 2 wheel bent for most of my commuting, and a velomobile for when the streets get sketchy, and it gets cold. It can be made fully enclosed for really cold weather, and with 3 wheels, I don't have any issues with a little snow or ice. If there's a lot of snow on the ground, I'll just drive, but even with the really bad snow we had this past winter, they do such a good job salting and plowing that I don't think I'll miss too many days do to weather.
