Brake pads press against the discs to slow the car, and they wear down with use. Replacing them in time protects the more expensive parts.
It varies hugely: roughly 25,000 to 70,000 miles (40,000 to 110,000 km), depending on how you drive, where you drive and the car. City driving and heavy braking wear them faster.
Look through the wheel spokes at the pad. If the friction material looks thinner than about 3 mm (1/8 inch), it is time to book a replacement.
Pads left too long can score the discs, and a pad change becomes a more costly pads-and-discs job. Worn brakes are also a safety risk.
Normally in pairs on the same axle. Front pads usually wear faster than rear ones.
Some squeaking is normal for a few days while the pads bed in. If it continues or turns into grinding, have them checked.
General guidance, not professional advice. Intervals are typical ranges and vary by car, so check your owner’s manual and ask a qualified mechanic if you’re unsure.