I do this with lots of things, too. Laundry detergent, dishwasher soap, shampoo, bleach, vinegar, paper clips (I was curious).
At work, I have been using the same box of No. 10 regular envelopes (500/box) for over 3 years. My box of staples has been around since Sept 2007.
After a while, I just know how long something lasts in our house or in my office.