from a old e-mail from Aqua-safe:

When to change the filters:
PRE FILTERS: We recommend changing the Pre filters every 6 or 12 months depending on use. If your average use is approx 2-3 gallons a day then you would change them every 12 months, if your average use is approx 5-6 gallons a day then you would change them every 6 months. or Adjust accordingly for a higher average use ie. for 300 gallons a month you would need to change the filters every 3 months.

POST FILTER: The Post Carbon Final filter for taste and odor only needs to be changed ever other time from the Pre filters.

DI FILTER (Maximus and Aquarium model only): If you have a DI filter (Refillable Clear Canister with DI Resin beads) you change it when the DI completely exhausts (changes color), how long it lasts varies from system to system depending on your use and TDS. If you have the older style white disposable DI filter you change it when the water starts tasting funny or the TDS goes up.
The 6oz DI canister lasts the average family approx 3-4 months.

RO MEMBRANE: The membrane generally lasts 2-5 years. We recommend manually flushing the membrane once a month along with changing the filters every 6-12 months to prolong the life of the membrane.

That depends on how much water you're making and the initial quality of the tap water. In Vancouver where the TDS hovers around 10 ppm people can get away with not changing filters for many months. Calgary has a much higher TDS so you would definitely have to change filters at least every couple of months to maintain the best results.

Having a TDS meter gives you an idea of how clogged things are.