from new they have about a 2mm ridge from the taper .When they are worn it will be very close to the taper.
I use the SPM/Werks clutch with 2 red and 2 black shoes and it lasts very long ,I went almost 10 months of use and no problems.
Thats 1 club meet a month ,1 practice day a month and 3 national events (3 days)
so its a fair bit of running.You would change clutch bearings more often than shoes.