You know I think it completely depends on type of ice pack, how it is wrapped-padded, where it's placed exactly, multiple factors. I know my tolerance level fluctuates and it is usually around the 25-35 minute mark. It seems to go from a pleasant pain alleviating chill to deep,deep cold ache in a matter of seconds, not something to ignore. Then off it goes.
I sleep with my pack and it's on/off multiple areas as needed throughout night. Either that or I get no sleep. Yes it holds the freeze well over night.