Shelf life for Novolin N?

Newbie question: the doctor and CDE said to toss the NPH 30 days after first use, but I just read the package insert, and it says 6 weeks (42 days). Better safe than sorry and all that, but this stuff is expensive – and surely the manufacturer has the greatest incentive to tell you to toss it sooner, so you have to buy more…

The general rule of thumb for opened insulin is to discard after 28 days. I currently use Novolog and [fortunately?] the last fill of my reservoir is often on the 28th day and what I pit in the reservoir lasts 3 to 4 days and I haven’t noticed any deterioration.

It could very well be, from what the manufacturer states, that shelf-life has been extended and us “old-timers”, me, doctor and CDE, haven’t up-dated our thinking. I’m going to reread my insulin insert and ask around.