We normally change the resin when it gets above 005. Takes just a few seconds to check it every morning. Can't remember what resin we use, buy it from Stream, think the last lot was about £68 +vat (free postage)
If you are running an RO then you also need to be changing your filters about every 6 months, our water from the RO into the tank is normally about 010, this is from a tap water reading of 260-270, that's going though a 40" RO and pre-filters