How long can a man remain in freezing water

The weekend beckons, and as with the end of any given week, I feel that there is something to celebrate.
So, to that end, I have set my 92 year old grandfather a challenge. If he can remain in a barrel of icy water for the entire weekend, he wins a ham. I shall have my servants refresh the ice periodically.
If he loses, by which he gives up and gets out, I shall therein forbid him from living at the manor and he will have to spend the rest of his life as a wandering vagrant.
From a physiological point of view, what are the fellow's chances?

Water temperature: 32 degrees or below
Time until exhaustion or unconsciousness: Less than 15 minutes
Expected time of survival in the water: Less than 15 to 45 minutes

Water temperature: 32.5 to 40 degrees
Time until exhaustion or unconsciousness: 15 to 30 minutes
Expected time of survival in the water: 30 to 90 minutes

Water temperature: 40 to 50 degrees
Time until exhaustion or unconsciousness: 30 to 60 minutes
Expected time of survival in the water: 1 to 3 hours

Water temperature: 50 to 60 degrees
Time until exhaustion or unconsciousness: 1 to 2 hours
Expected time of survival in the water: 1 to 6 hours

Water temperature: 60 to 70 degrees
Time until exhaustion or unconsciousness: 2 to 7 hours
Expected time of survival in the water: 2 to 40 hours

Water temperature: 70 to 80 degrees
Time until exhaustion or unconsciousness: 3 to 12 hours
Expected time of survival in the water: 3 hours to indefinite