This is a perennial topic of discussion. In general, insulin can last much longer than what the manufacturers say. It depends very much on how it's stored and where, of course. but those date ranges are generally very conservative. We've had a lot of discussion here on this subject, and many people find that insulin can remain effective for a lot longer than the guidelines say. In my case, for example, I keep all my insulin refrigerated all the time, opened or not. It's not at all uncommon for a vial to last me close to 90 days.
Everyone's experience is different, of course, but the rule of thumb is that you can usually do better—often considerably better—than we're told.