Most pads have a small metal tab that squeals when the pad is nearly worn. If you hear a high squeal when braking, book a check within the next week or two.
Grinding is more urgent: the pad may be down to its backing plate and cutting into the disc. A longer stopping distance, or the car pulling to one side, also needs a look.
We measure pads and discs at every full service, and the visual brake check is free with any booking (sample).