Once ATC gives the, "go ahead" obviously that means the pilot CAN go. Does anyone know how long he/she has to pause before they MUST go? Is it essentially expected that they immediately go once given clearance? Presumably, ATC is timing things and is expecting the pilot to be out of the way shortly.

I suppose it could be different if the same runway is being used for take-offs and landings whereas one used just for take-offs wouldn't be as critical.