they work all the time when the engine is running ( i of course assume u are talking about the engine mounted mechanical driven pumps) and not at maximum rate - maximum rate 100% of the time is not possible because the speed of the pump is determined by the speed of the gearbox and therefore the engine- at any given speed the fuel pumps supply more fuel than is need - this excess fuel is then bled back toward the engine pumps via actuated valves and not by preset pressure relief valves