I have a sweet little tool that tells me when it is time to replace clutch discs. It is called a digital vernier caliper. The life of the clutch can't be just pegged at a certain time for replacement. Too many variables. The 2 biggest killers of clutch discs (other than heat) are pressure plate springs that get weak and the other is not using the correct motorcycle oil for the engine. Just any old 20w-50 oil is not best for your engine because they do not have polymeres that hold the molecular structure of the oil together under compression after it goes through the gearbox plus most 20w-50 oils not formulated for wet clutch applications like missing friction inhibitors and detergents that are safe for the clutch. Take like Castrol 20w-50 oil, put it in the engine and the oil will break down from compression between the gear teeth and will leave deposits baked on the clutch discs which cause it to slip. Slipping clutch causes heat and that in return makes the clutch springs lose tension. I run the set-up like Jim does and have had never needed to replace a clutch disc pack with regular oil change policies to keep the oil in the clutch fresh.
~Gimpster~