I would use them until you notice that your #s are getting higher than you expect them to be. That is as long as you've kept them stored in the fridge.
But then, I'm known for being cheap. I even used my Lantus 4 days to 2 weeks past it's "Proper Time." I just kept a Very Close eye on the #s.
The standard "Acceptable" plan is to pitch it once it's expired.
Also, when you went through Insulin school they should have warned you to always check the expiration date of your insulin so that it wouldn't expire on you.