Brake pads are replaced if...
- Pads have tapered (worn more at one end)
- Pads have chunked (broken up perimeter
- Pads are below about 3mm or higher if long service interval
Brake discs & pads are replaced if...
- Braking pulses noticeably due to warped brake discs
- Brake disc has blue, cracks, deep grooves
- The latter tends to suggest frozen caliper (buy remanufactured)
Some cars are hard on their brake pads - others hard on their brake discs. That is even with normal town driving rather than "spirited" - it comes down to vehicle weight, rotor size, pad size and so on.
If the service interval is 12,000 miles you may be ok on the old pads. If it is 24,000 miles on brakes you may not be - however most garages comment at the MOT if pads are worn.
It might be worth doing a Google for basic car maintenance - just to understand the basics when dealing with mechanics, that way you know when to be suspicious or not. Specifically what brake pad wear looks like, when replacement is due, what brake disc wear looks & feels like, when tracking is out re tyres or not, when a clutch is going & how to test a clutch. Otherwise you look like a wallet and will get treated like it - not by all mechanics, but some.
A good place to ask car questions is the technical question of
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which has ASE L1 Master Techs. They can pinpoint specific gotchas with a car, be it an old student car or the latest offering.