I can vouch for numbers letting you know that your Lantus is not working.
I recently started a cartridge, my first as I was using a huge vial that I was sold by the pharmacy, and boy did it ever make a BIG difference in numbers!
I have been led to expect any opened vial/cartridge to be good for around 28days or so, and the unopened ones, according to the pharmacist, should be good till that official expiry date on the package.
But again, you can take that as likely, not definite. I bet a cartridge could last longer and be fine, or go bad before the 28days, or even be bad when you open it. The numbers should tell you what you need to know.
As with any expiry date, I would not use the item if it has expired, even if I was told it should still be good, especially when it comes to something medicinal. I toss expired sinutab and Contac C and cough syrups too.