How quickly do brake disks wear down?

I want to share an experience I had and get your opinion on this. Recently, my husband and I moved back to his hometown. Not having any relationships with mechanics here, we decided we would use the shop my mother in law has used for over 30 years. She has a very good relationship with them and we know they treat her well. We therefore felt they were trustworthy.

About 2 weeks ago, my car went in for its service and then for its roadworthy test. It passed with only minor repairs needed. We had heard a funny noise from the engine every now and then before and had mentioned it to the mechanic. When we came to pick the car up, he said they couldn't see anything so no one was any wiser where the noise was coming from. We got our keys and were told to come back in 15,000km or one year (whichever comes first) for our next service.

For the last 2 weeks, the car didn't make the noise at all. So we figured whatever it was, the standard service had solved it.

However yesterday, the car started making the same noise again. But then it was non stop not only every now and then. We brought it straight into the shop and this morning I went to pick it up. It was a problem with the throttle which was fixed quite easily and cheaply. The noise is not happening anymore.

Anyway, while I was there this morning to pick it up, the mechanic (a different guy than 2 weeks ago) mentioned that he noticed my front brake disks needed replacing and recommended I make an appointment to do it. He was surprised when my response was to ask why this was not mentioned 2 weeks ago. He couldn't give an answer. He then told me the disk change could probably wait a while. I asked how long, because the service and the road worthy test are meant to say that the car is expected to drive safe and no major repairs are expected to be needed in the next year. He said he couldn't say how long, as he doesn't know how much I use the car. I then asked if he thought it would be ok for another 15,000km as per what I was informed 2 weeks ago. He didn't really answer then instead told me it depends on my driving style and it could probably wait until the next service.

I fully accept that they can never guarantee there will be no problems for the next 15,000km / year. But this all seems slightly dodgy to me. 2 weeks and not many kms later and suddenly I need my brake disks replaced? Now I am trying to figure out if this mechanic was trying to rip me off or if he was trying to cover up for the mistake of his colleague in not noticing the brake problem last time. Also, why was he looking at the brakes when the noise was caused by the throttle? The noise didn't happen when the brake was pressed so I don't understand why they were looking there.

My gut feeling is this guy is trying to rip me off. The road worthy test is an official legal governmental test, the brakes should have been checked then. But because it's the brakes, I don't want to take any risks. I did some research online and found you could check for ridges on the brake disk yourself. My husband did this. There is a ridge there, not so deep but it's there. The car brakes fine, no shuddering or moving to one side.

My husband is bringing the car in this afternoon to another garage for a 2nd opinion, but how do we know the 2nd garage is trustworthy? They might also see the opportunity to tell us we need the brakes done and make some money. Or, as this is a small town, they might agree with the other place just to have their back.

Any advice on this is appreciated.