How long do the Varilok clutch plates in a 2000 Jeep Grand Cherokee last ?

Almost forever!

Clutch plates in a Gerodisc setup do not have any preload (like all conventional limited slip units). Hydraulic pressure is only applied when slippage occurs. The clutch plate below was pulled after 100,000 miles of heavy duty use - you can still read the parts number printed onto the friction material !!

 Differential with Gerodisc