Q&A

Any time a brake job is performed, brake rotors should be measured for thickness. Each manufactuer has a minimum "discard" thickness for rotors that is specific to the model for maximum heat transfer and durability. To machine a rotor and install it under this thickness is usually illegal and unsafe. From my experience, brake rotor wear varies most typically from driving habits. On the same model, one driver's rotors could last 200k miles while the next is 50k miles. Completely worn out brakes (metal to metal) will usually destroy rotors in a manner of a few miles though.

3mm is pretty darn thin. 2mm is about as low as you should go. It's always difficult to tell how soon you will absolutely have to replace them, because your 60-90 miles a day could be start and stop and bumper to bumper traffic, or it could be all highway miles and few stops. Or a combination.

That said, I would not let the car go more than 1000 miles or so at 3mm. That's just a ball park figure.

How many miles in the car? Did they tell you the thickness of the rotors also? It's an important information because they could be resurfaced (machined) if within specs. The big chunk of the $555 are the rotors.
3mm is very low, if you keep it driving sooner or later you will hear a metallic grinding noise when you press the brake pedal. That means the friction material is gone and metal runs on metal.

Zee