The standard is 30 days to expiration once an insulin vial has been started, whether a 10ml vial or a 3ml cartridge in a pen.
However, the shelf life unused while refrigerated is usually around 18 months (judging by the expiration date on my vials).
This got me thinking: What about a pen that is kept refrigerated, the end cleaned with alcohol before each use, and a new, sterile pen needle used every time... Shouldn't that pen be good for the full 18 months if kept refrigerated?
The 30-day timer is based on two main issues: Degradation of the insulin being stored at room temperature during active use of the pen, and introduction of contaminants when inserting the needle into the insulin in the cartridge.
Keeping it refrigerated takes care of the first, careful sterile procedure covers the second.
My reason for interest in this is I've been adopting intramuscular injections for doing corrections using disposable syringes. I'd much prefer to use a pen for this, however it's unlikely I'd use up a whole pen in 30 days since I pump for most of my insulin needs. What I'd like to do is just leave a pen in the fridge at home and at work, which would cover the vast majority of locales where I do corrections. If not at either place, no big deal -- just correct subQ with the pump.