When I filled my prescription for NPH, the pharmacy tech said it was good for 28 days. The patient insert, however, stated 42 days. I have so far used it for 42 days. I think maybe I've seen a loss in potency in the last week but am not sure. Looking for definitive answers, I found this non-answer of an article:
In which none of the manufacturers advocates 42 days, one says 7 days (!) and much evidence is offered that the decline in potency of Lantus over time is more noticable than that of NPH.
When I personally tried using a Lantus vial (out of the fridge) for as long as I could, it was day 53 when I gave up on it due to very high blood sugar for the previous two nights. I have used Novolog past five months and it worked fine; I've used a vial of Novolin R for about two months with no problem.
Now I guess I'll give the NPH a whirl and see how long it lasts... maybe? I keep going back and forth with myself about whether I should bother.