Never. I usually have a little bit left over at the end of each month. Manufacturer says to toss it after 30 days of being punctured I tend to follow that rule pretty strict.
I do the same thing and pull the insulin out of my insulin pens! I thought I was the only one, and my CDE had never heard of anyone doing it either. I use such a small amount of insulin, that I would waste half a vial a month. It saves so much money not to have the entire amount of insulin opened at one time and I have one copay for 1500ml instead of 1000ml.