As part of a heat treatment process, cylindrical, 304 stainless steel rods of 100-mm diameter are cooled from an initial temperature of 500ºC by suspending them in an oil bath at 30ºC. If a convection coefficient of 500 W/m2·K is maintained by circulation of the oil, how long does it take for the centerline of a rod to reach a temperature of 50ºC, at which point it is withdrawn from the bath?