The rule of thumb states that without any addition of gel packs, the food kept in an insulated lunch bag should still be warm even after 2-3 hours, and if it was kept cold, meaning it is highly perishable like yogurt, it should last for about 4-5 hours.
But when the gel pack is incorporated, the shelf life will exceed 3-4 hours. All these will depend entirely on the surrounding temperature and the quality of the insulated lunch bag. Thus it is advisable to test your lunch bag before taking it outside.