It depends on what contaminants are in your water and of course how much of each there is. If you are not sure, then the easiest option is to use a clear holder with a colour changing resin. Typical "example" replacement times, quoted by suppliers, are 6 months to a year to replace, but you need to be sure of when to change it which you can do when your output TDS starts going up (in accordance with the resin manufacturers instructions) or when the colour changes if using a colour changing resin.the resin that is...replacement time
If your water is proving to be terrible and you look like you are changing too many resin canisters, then I saw on you tube that BRS do a 3 cartridge usit that contains 3 different types of resin (allowing you to only replace the resin type that is being consumed and as BRS would say, saving money in the process.
I have no experience of individual resins, but I expect someone here has.