An insulated bag will keep food hot for up to 3 hours.
If you have any hot foods in the insulated bag it should be served or eaten within 3 hours because after 3 hours the food will start getting cooled down.
But as long as you eat or serve the hot food from the insulated bag within a few hours it will remain hot enough to eat or serve.
So when preparing hot foods that you want to remain hot until serving it's best to have it prepared about an hour or 2 ahead of time and then put it in the insulated bag right away or keep it hot in the oven or wherever you cooked it.
Then place the hot food into the insulated bag when you're ready to take the hot food to wherever it is you're going.