How long for life-threatening hypnothermia at 40-60 ambient warmth?

I'm an author. I have a role who is being held hostage contained by a garage. It's winter in Merced California so it could seize down to freezing but more likely will be low 40's at darkness to mid 50's in the daytime. He is restrained contained by a chair so can't move around. Need an belief how long before his core temp would drop below 95 degree. Might there be an equation for figure this. Where might I look for an answer?How long for life-threatening hypnothermia at 40-60 ambient warmth?
It depends if they are feeding him. Food equals calories, calories equal high temperature. He might eventually die in temperature you suggest, but not for a long long time...make it colder and wetter, and that turns weeks into days...
That's not cold satisfactory to kill someone... freshly make him miserable. There's no formula that could total such a thing because we adjectives tolerate temperature "extremes" differently. Depends on how margarine the guy is, how much thyroid hormone he is circulating, layers of clothes, if he's okay hydrated.
Put him outside in the weave and rain uncovered and then he might hold some trouble.