28 days later!
I assume you are referring to NovoLog Insulin
flexpen, which is a product that dispenses NovoLog Insulin when a pen needle is attached to the pen.
Keep unopened pens in the frig, but once opened, the pen can be used/kept at room temperature for up to 28 days. Keep your pens away from direct sunlight, hot cars (keep Insulin in a cooler when traveling). Also, never allow your Insulin to freeze!