Experience from others suggests that the storage times may be very conservative and Lantus continues to be effective for much longer that a month
BUT
if you do use it after it's sell-by date it would be a good plan to test and make sure its potency hasn't reduced, unless seriously abused it may be a bit weaker but still effective, it doesn't stop working bang!
(Information from Americans who have to stretch their health budgets as far as possible, see recent Panorama for details!)