That would depend on the tyre compound in question, car, your driving style and what you have the tyre wear rate set to.So basically I should keep putting laps in until I notice a slow down? I don't really watch gt3 racing, how many laps can they generally do around brands Hatch? Is it like 5, 20, 50 or 100?
Personally I have it set to real and have never had tyre's degrade to the point of being useless in the dry up to 20 odd laps. The ones to watch are Wets when the rain stops, these you can fry to the point of being useless within two laps of the track going dry.