My Hydro Drip system gets changed every week. I Was told by the guy at my hydro store I could go 2 weeks with the nutes I am using. but its supposed to be every week. I think it has to do with yer nutes.
When the plants are small and drinking alot more water then eating food every two weeks is fine but when they start eating alot of food I'd suggest changing them once a week but save yourself sone nutes and wait till they start eating them