How long can a car be out of oil before permanent Damage is done?

[exhampriory]

How long can a car be out of oil before permanent Damage is done?

My wife and I just borrowed our Pastors car. He dropped it off and when we were taking him back to his house, I could hear the Pistons clacking away worse than I've ever heard on a car that is low on oil. He is a pretty Old guy and can't hear these kinds of things. I took it straight to a Gas station after dropping him off and found that the oil tank was bare. The dipstick was bone dry. I dumped a couple bottles of oil in, the clicking ceased. I just want to know what the chances of permanent damage are as this car is probably going to be mine in a few months so I am worried. I DO know that the car was running impressively perfect about 5 months ago. Lately it has had quite a bit less power/acceleration from stationary.. Any info would help.