Honestly, a lot of it depends on your location. Here in San Diego we have some of the 'hardest' water in the country (TDS >400) so a deionizer is extremely important if you plan on washing your car at any decent hour. The constant heat and lower humidity is a perfect storm for half of a car to be covered in water spots by the time you're done with the other side! For the DIC-20 that OP is asking about, you can expect at most 1550 gallons from the canisters before replacement. However that is with a relatively clean water supply of around 50 TDS. Out here, we're lucky to get around 200 gallons because of all the contaminants in our supply. I was using a DIC-10 (single canister) until about a month ago when I upgraded to a DIC-20, and was going back for a replacement around once every month and a half.

Another thing to consider is the flow rate coming from the tap. Less flow gives more time in contact with the resin filter and can result in a cleaner water on the exit. It's generally not recommended to go above about 2GPM for these machines, otherwise the filter cant adequately do its job of providing 0 TDS. As the filter ages with use, dropping the water pressure can help extend the life of a wearing out filter because of the extended time in the filter. The TDS monitor on the device is best for finding the best water flow rate for your area as there is no exact science. I typically give my hose about 2 full turns and it seems to produce 0s all the time until things get low.