I'm not sure if there's a way of measuring exactly how much CO2 is needed to condition a single keg. The 2 gauge regulator I have really only tells me the tank is full or when it's just about empty, since the gas continuously expands to fill the tank as it empties. However, the owner of my LHBS told me one tank should last up to 6 months, even running 3 kegs at a time, so I would think one keg probably doesn't use much gas to condition.
At least you didn't go off for a conference for a week and come back to find a previously new and full CO2 tank completely empty and no pressure on your kegs. That's what happened to me yesterday.
Not just 1, but 2 leaks found after the purchase of a new CO2 cylinder. Hopefully all is fixed!