How do you know when they need replacing?
stick your fingers in to feel the surface of them... if they have any noticeable grooves on the surface, or a noticeable lip on the outer rim then start thinking about new ones... big grooves could potentially be very dangerous (unless they\'re meant to be there for cooling LOL)
Uprated disks (slotted / drilled) may have big performance drop off after some wear and tear.
Depends how they should handle when new - do they need warming up? should work well from cold?