the changing of the RO membranes are dependent on several things, such as the back pressure build up over time and the flow rate over the membranes pending the type of system you are using.
the pressure differentials over the membrane will give you the indication that membranes must be back washed, you will also see that the permeate decrease and re-tent-ate increase.
If you have have a CDI and inline conductivity system you may also see that there may be an increase.
You should not really do it in specific intervals as it is dependent on the amount of RO water you
produced for use., It is also critical that the RO system be sanitized on a regular basis to prevent
possible grow through on the membranes, Siome system you can do RO separately as it has a heater tankwith it own heating if you run a hot system tanks loop etc is bat of this this is maybe not such a big issue,
However if you run a cold system normally 8 degree C you should do this for on fix intervals