Change you pre filters according to the amount of water that passes through them. Not all pre filters are the same, when you buy them explain the system you are running, your tap water tds and the amount of water you use and the supplier should reccommend the correct filters and tell you the amount of litres they can filter before they're spent. RO man can anyway.
The carbon filter is so important to keep fresh as this is the one that takes the chlorine out of the water. RO membranes have zero tolerance to chlorine and it can ruin them very quickly.
All information given has been acquired from bitter experience!