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I don't know that there is a set mileage due to each persons type of driving, all highway, city, stop and go, hard braking. Me personally, I wouldn't drive more than a couple of hundred miles but then again I'd never wait till I heard the sensors. I'm at 30k and I know there is meat left on the pads but I'm going to order rotors and pads soon and replace them sooner than needed.
Am I missing something here? You have been given answers, do you not like the answers you have been given? No, there is not a idiot light that lights up on the dash, no, Lexus is not going to call you because the computer called them to say you needed brakes. Either look at them yourself, take it to have it looked at or wait till you hear the squeal and then you'll know.
All you need to do is follow the instruction manual or what your local Lexus dealer tells you and you will be fine. You own a Lexus, you don't need to decipher anything for yourself. Period.