That is a long subject, dependent on many variables (hydration, exertion, air or water temp, wind chill, wet or dry clothing, time of last meal, blood loss from trauma). If we are talking hypothermia only the speed of demise primarily depends on the temperature differential the person is experiencing on their skin. You can become hypothermic by swimming in a lake in July if the water is cool enough, there is a nice breeze and you stay in for some time.
I don't want to say much more for fear of being 'spanked' for being off topic. I already got spanked yesterday.