If you read up on the % CO2 left in the cylinder when the gauge starts to drop, you'll find somewhere between 90 and 95% of the total CO2 gone. I think it's in the 96% range or so, but others slightly disagree.
In any case, minimum 90% of your CO2 is gone when the reading begins to drop. You've gotten 90% of your money's worth, which was dirt cheap to begin with. Just refill as soon as you see the drop and save yourself the potential catastrophic series of events which begin with you figuring you have at least a couple days left of CO2.