Insulated lunch bags will keep food warm for between 3 to 5 hours depending on the quality of the lunch bag.
The insulated lunch bag won't keep food too hot for too long so you should eat the food within 5 hours at most if you want to eat the food warm.
Regular lunch bags or lunch boxes will only keep the food warm for an hour at most or even less than that so if you have a regular lunch bag or lunch box that is not insulated the food should be eaten within 30 minutes to an hour.
But the insulated lunch bags help to retain some of the heat in the food for at least a few hours but after a few hours the food starts cooling off.
In my experience with an insulated lunch bag the food was still somewhat warm after 5 hours but not steaming hot.