No special time. You should always machine the rotors to do a proper brake job. If the rotor thickness is below specifications it must be replaced. With the new metallic brake pads, you don't just throw on a set of pads. The rotor surfaces must be prepared.
〉 Answered on Dec 10th, 2012 by Jo An Holt, Automotive Journalist at Holt & Associates
You need to monitor the wear of the brake pad material. You can do this by safely removing the wheel/tire, and inspecting the brake pads to assure they are not worn down to the scrapers or to the backing plate. Modern brake pads have a small metal tab which will begin to lightly scrape on the rotor and make a metal to metal sound when the brake pads are worn.
〉 Answered on Dec 10th, 2012 by Karen Salvaggio, Owner - Professional Race Car Driver at Thunder Valley Racing