Most insulated lunch bags keep food cold for as long as 3 to 4 hours.
You can help keep the food cold longer and make sure the food stays safe to eat by adding frozen ice packs to the insulated lunch bag.
Most people will eat the food in the insulated lunch bag well before 4 hours so it's usually not an issue.
But the insulated lunch bags I've had and used at work or took on road trips always kept the food cold enough for at least 4 hours.
After 4 hours the food was still cold and safe to eat but it was losing it's coolness.
So as long as you eat the food in the insulated lunch bag before 4 or 5 hours it should be fine.