Bike has suddenly started to squeel and vibrate with rear brake on when braking very hard for some time, gentle JRA it’s fine. The only thing that is worn is the rotor, definate ridge can be felt.

How often do rotors need changing?

Wally.

It will judder if the pads are running over the raised arms of the rotor, I have one like this. But equally it could just be contamination on the rotor/pads (oil/brake fluid)

Yes and when they wear out in answer to your two questions.

As noted above contamination could also be the issue.

I’m sure someone will be along shortly to tidy this up but worn rotor would be down the list check for me

wheel inserted correctly
caliper alignment
pads correctly inserted
rear hub bearings
rear sus’ bearings

Pads are running over the raised arms and cut out quite a groove. New rotor and pads time. Atigee – I will check those also, but I think they are all good – Having changed all bearings recently.