OK thanks, so a little different to cars then in terms of preventive maintenance?
I've done about 1500 miles on it so not huge mileage, but a lot of descents with frequent heavy braking on my commute during all weathers etc. I have no idea how long the pads should last so think these probably ought to be changed.
You need to measure how much material is left, if the pads have at least 1.5mm they are ok.