I have ACC and have tested this with a follow-car and cellphone
The car acts just like it would if you operated the brakes. As soon as you feel the car brake the lights come on.
I've also been checking for my self when driving at night. You can often see the reflection of your brake light light up road signs behind the car and from my experience the brakes is activated almost every time you see the engine go into "regen" mode. (Blue efficient dynamics bar in the cluster)
So to sum it up... when you feel the car brake the lights come on. So in some situations the following car will see your brake lights a little more often than if you operated the car yourself. And yes... cars with ACC will eat the brakepads a little faster than cars without ACC.