keep the intake on 24/7 but the exhaust only when the lights are on. Some people like to leave the exhaust fans on for an extra couple mins after the lights turn off so that no leftover heat stays in the room during the dark period
no need for an intake fan a passive intake works best espetially if you have a carbon filter... usen both fans as exhaust is the best option i leave mine on 24\7 all summer long and in the winter i put it on the timer with the llight