Pete,there is not a lot of meat on the plates when they are new.There is a measurment for the minimum wear thickness of the plates;
SP,SPS and 996S models,standard 2.5mm.Service limit 2.3mm.
All other models,standard 3mm,service limit 2.8mm.
The general rule of thumb is "if it aint broke,dont fix it!"
If the clutch is working ok,ie not grabbing,slipping,dragging or making exessive noise,.......then leave it alone!
Seriously tho,its good to clean the dust out every now and then .Check the plates for signs of overheating(turning blue) or scoring.Check also the clutch outer drum for signs of wear where the plate tabs are hammering the slots.The plates should'nt be warped either.
That is all!