How long does it take to get frostbite in -6 degrees weather?

Various factors . General health, baseline circulation, smoking status, core temperature , improper or too tight clothing, nerve issues, metabolic issues, wind chill, wetness, use of beta blockers all affect the time frame. Most important if you are getting frostbite, and are not in a secure location avoid rewarming true frostbite and then refreezing again. Do not wash hands in hot water. Do not drink or smoke.