I have tried this. -20c and i went and tried to sleep in my car with the windows down a bit to stay out of the wind. Lost feelin in my feet and hands before my roommate noticed my shoes and keys were gone but car was on driveway. I dont really remember them coming out to get me or anything, just the pain when my hands started to feel again. it hurt worse warming up than it did getting cold, and that is very painful for the first while.
Main factors are cold and time. Your body is 36c ish and bringing it down even to 30 can be very dangerous. If you go outside in 4-10c, you could die in a few hours if you were naked. If you went out wet in that temperature, you could pass out in 2-30 mins. So anything below freezing will speed the process up. Therefor there is no reliable timeline. Too many factors from what you wear, your weight, the temperature, humidity, wind.
Hypothermia has a pain threshold that is a good indication if its working. When you start, its painful, shivering and tensing up. After a while, you stop shivering and it just hurts a little. Then you start to feel warm. A lot of hypothermia victims are found naked because they thought they were sweating and probably delirious. Then you fall into exhaustion. If you don't get delirium, and save yourself by going inside, you can expect to drift off to sleep pretty painlessly from here.
This is a bad way to try, very long process and SI is stronger with the pain. I had to drug myself pretty heavily.
