As long as there's no salt, I usually hand wash at 32 degrees and above. I strictly adhere to my once a week routine. If conditions become extreme (like last winter), then my line of defense may include:
1. Washing once a week (rain or shine, no buts). I accomplish a lot with this routine.
2. If its below 32 degrees, then the car will stay dirty until it reaches 32 and above.
3. If you can, have the car hand washed at a facility if its too cold out.
4. If there's salt, and its been more than 4 days, and its still freezing out and/or your schedule doesn't permit time for handwashing, then plan to hit the carwash as early as possible. I don't recommend the automated places at all, but its better than letting lingering salt stay on the car's body.
5. Be determined. Rust and corrosion still exists, even with galvanized steel. The aim is to have less to correct in terms of paint issues when detailing season restarts (esp. for the northeast region).