Hi Bluejean. It depends. How long does a cartridge of insulin last for you? How hot does it get where you live? How long do you leave your infusion sites in before you change? All of these can have an effect on your blood sugar levels.
I use a cartridge of insulin every five to eight days. It seems to be okay for that long. Also, I took unrefrigerated insulin to Europe. I was gone for six weeks and the insulin worked fine the whole time.
I sometimes don't change my infusion sets as often as I should and my bg's sometimes rise by late on the forth or on the fifth day. If I change the infusion set, but not the cartridge, the insulin seems okay for the entire time.
I don't think the pressure in the bottle should have anything to do with it. The pump rod itself pushes the insulin out of the cartridge and into the tubing. They are made to deliver at a steady rate and would not likely lessen in pressure as the cartridge empties.