The best way for me to tell it's time for new fiber plates is by powering up a big hill.
The original steel plates shouldn't go bad and I use stock springs (easy on the hand )and fiber plates, I have a newer style clutch release with ball bearings that I may install this Winter.
I'm getting ready to swap out the gears for a fresher set and while I was inspecting the clutch basket I was reminded that the clutch plates dig into the basket over time and leave grooves and burrs in it that may hang up the plates, you can feel them by running a finger along the slots,( be careful not to cut your self)
I have a points file that I use to clean this up, works great!
I have a small square stone that I use to take any burrs off of the inner aluminum " clutch hub"
Plus 1 on Rotella T4 10w40