With my insurance, I only pay the co-pay. If I used twice as much insulin, I'd still pay the same. So I have no problem just throwing away a nearly full pen or vial. I'd rather have assurance that my insulin will work when I need it. And I often carry my humalog pen all over, with less than ideal assurance that I've kept it at "room temperature." I know I've burned my pens before. And basal insulin's like Lantus are even more fragile.
The best I can tell, the manufacturers did studies of how well the insulin stood up to worst case conditions over the time, and pretty much across the board, we have the same stated storage and in-use claims. Refrigerated, insulins are good through the expiration date, often up to 2 years. Once started, vials or pens can be kept safely at room temperature for 28 days.
So what does this mean? That the testing showed that the insulin stored at like the warmest temperature was "always" fine at 28 days. If didn't show it failed on day 29, but the manufacturer makes no claim after that. If you search, you will find of other discussions, like Clare's. A detailed discussion of what the insulin manufacturers say on the topic can be found in this editorial in Diabetes Care.
ps. And don't forget BSC's Top Ten Indications You Insulin Has Gone BAD!