If refrigerated, how long can you continue to use a bottle of insulin - any type?

At the expensive co-pays for insulin, I'm wondering whether you can use the bottle until it's empty, if it's refrigerated - rather than dispose of the bottle after 28 days?  And, why do you need to dispose of it after 28 days?  Does it loose its' potency?  I realize that punturing the rubber tip introduces air into the bottle but does punturing the rubber tip breakdown the insulin, in any way?

I'm just wondering whether "old" insulin is the reason that I may have high blood sugar readings.  I don't always have high readings but sometimes I do & I can't figure out why?  I'm on a very strict diet & adhere to it.  I test my blood every two hours during the day because I suffer from Hypoglycemic Unawareness.  I inject insulin four times daily before each meal & before bedtime.

I'm concerned whether I'm hurting myself by not disposing of the bottle after 28 days & keeping it until it's empty.

Would appreciate hearing from some doctors, experts & fellow diabetics, on this subject.

Vikkie