I usually set them to backwash every four days. Because there are so many variables involved in this, it is going to be hard to give you an exact time, but because it's so cheap to backwash, I like to err on the conservative side. It might be interesting also to see if you have a lockout switch on its head. This prevents the RO from making water when the filter is backwashing. Since they usually backwash at 1 or 2 AM, it's not normally a problem, but if your system is making RO water during the night, you should consider it. The switch is under $10.