Technically they say 28 days w/o refrigeration and around that when opened, but I think this is so they sell more of it. In my experience, it seems to keep working (I'm diabetic) for a long time. I carried one bottle around all summer in the heat and it seemed the same as new. I could tell with a blood sugar test if it goes bad, I'd assume anyone could, but I'm not sure on that....
thanks guys , some info from http://www.drugs.com/humalog.html
Vials and cartridges of Humalog can be kept unrefrigerated for up to 28 days, but should not be exposed to excessive heat or sunlight
Once punctured, the insulin vial or cartridge in use, whether stored in the refrigerator or at room temperature, must be used within 28 days. Throw away any unused insulin 28 days after the vial or cartridge is first punctured
can humalog be stored at room temperature or does it need to be kept refrigerated? ive read that when stored at room temperature it stays potent for a round a month , is this true? thanks guys