For most existing designs, magnetic fusion reactors are supposed to operate continuously, without need to stop for cooling down. For the easiest to achieve D-T fusion reaction, thermal energy deposited by fusion neutrons inside of the reactor wall (blanket) would be removed by active cooling and used to power a turbine and generate electricity. However, a fusion reactor would have to be stopped periodically, after operating for a ~1 year, for maintenance. The main reason for that is a high heat and neutron loading of the fusion reactor nuclear zone. The nuclear zone will be exposed to intense heat and neutron radiation, irradiation by ionized particles, and hard X-ray photons. This would severely limit the lifetime of nuclear components, for existing technology; replacing the nuclear components would require shutting down the reactor periodically. Note that ITER is not going to be a fusion reactor but rather a reactor-scale fusion experiment. It is planned to operate in pulses a few hundred seconds long.