Ok then, so pads are straight forward to work out when they need replacing, but how do you know when to replace rotors?

I’ve never replaced discs on a mountain bike unless bent, i don’t ride enough off road to wear them out. On my road commuter i replaced the (avid 160) rotors after about 5 years wear – which would have been over 24k miles riding. The rotors were noticably thinner in the contact area and were not working as well.

As above they do wear out but ime not much.

I replaced the G3CSs on my four year old Cannondale the other month and although the new rotors feel a lot better, the old ones were still OK after prob 10k off-road miles

I’ve worn out a few. I run Elixir R’s and I’ve found the G2’s and especially the G3 clean sweep’s produce low rev resonance when they’re on their way out. You can also feel a marked difference in the thickness of the disc splines and the braking surface. Heavy scoring is another reason to change ’em.