What type of insulin are you using?
From the packaging insert with my humalog:
Keep in the refrigerator or at room temperature below 86°F (30°C) for up to 28 days.
Keep vials away from direct heat or light.
Throw away an opened vial after 28 days of use, even if there is insulin left in the vial.
Unopened vials can be used until the expiration date on the Humalog carton and label, if the medicine has been stored in a refrigerator.
If your cooling packs can keep the insulin below 86F/30C then Eli Lilly "Garuntees" that it will be good for 28 days. Chances are that these recomedation are on the safe side and you can push these, but I cannot tell you how far you can push it.