Rotors in USA - brake discs in UK. It's expected to have very slight circular grooving on discs but not deep grooves. If soft pads are fitted they wear comparatively quickly leaving the disc relatively unworn. If harder pads are used, the pads last well and instead wear down the discs. If the thickness of the discs is less than 24mm, they should be replaced. When pads are down to 2mm, it's time to replace those.