I have cooked Pinnacle Peaches. How soon must I eat them before they will no longer provide the bonus for being hot?
Food seems to remain hot for 100 in-game minutes (8 minutes and 30 seconds in real time).
I cooked a can of Pinnacle Peaches, then cooked other items while frequently checking the status of the cooked peaches in my inventory. The peaches kept the Hot status until 100 minutes had passed in the game, at which point they became Cold.
I was standing indoors next to a fire for the entire time. I do not know whether colder ambient temperature would have an effect on the food.
Update: The behavior does seem to vary, probably by ambient temperature. I attempted to let hot soup sit on an outdoor fire barrel for 1 hour while I slept, and the food was cold when I awoke. I reloaded the save and tried to sleep outdoors with hot food in my inventory, and it was cold when I awoke.