I used my first vial of Lantus for almost a year. I kept it in the fridge on the shelf (not the door, not the veggie drawer)...and it was good to the last expensive drop.
Lantus for humans is very durable and the directions/recommendations are written for humans who carry it in their purses or otherwise go thru it quickly. With our kitties we use so little (well, most of us)...and it will last far longer than the recommended date on the side of the bottle. It's kinda like groceries with the 'sell by' date...that does not me the 'safe to eat/consume' date. I personally keep shelf stable items far longer than the 'sell by' date. I rarely use that date as the date to discard canned goods, sealed jams/jellies, pasta/rice, etc. As long as it's stored properly (and yes, nuts, flours, grains are stored in the freezer) and can last for years!
You can also purchase Lantus from folks in the Supply Closet here..often some very good deals there from other members.