If you're pretreating your water with a water softener (and an iron filter, if on a well with high levels of iron) your pre and post filter should only need changed once a year, provided your pre-treatment equipment is working properly, and sized properly.
Reverse osmosis systems that aren't used enough have the inherent tendency to a condition known as "TDS
Creep". Dissolved solids will actually work their way back past the membrane into the inner workings of the reverse osmosis system. When the reverse osmosis system is used the next time, either by opening a drinking water faucet, or, in your case, opening a valve up for make-up water, those dissolved solids work their way into your holding tank. Over time, those dissolved solids build up in the holding tank.
By completely flushing your system once a week, you'll accomplish some positive results in your product water. 1) You'll greatly decrease the TDS
content of the water in your storage. 2) You'll extend the life of your membrane by cleansing the outside surface of any mineralization that may start occurring.
, 65 gals. is an awfully big holding tank, if it is a reverse osmosis tank....(With an air bladder....) If it's a atmospheric tank with a float, I'd completely flush the R/O to drain, and then open the valve to allow the atmospheric tank to fill up.
More on TDS
Creep here: Reef Addicts - RO/DI TDS Creep - is it worthy of concern?