I have had a CR Spotless DIC-20 for about three years that I bought from Costco. I wash a Grand Cherokee, a Dodge Dakota and Benz SLK about once a month each, and have yet to replace the resin. The cars come out 100% spotless, even the dark tinted windows. Also, keep in mind that rinsing with distilled water means that you never touch the car during a drying step. This prevents or eliminates 99+% of swirls marks and scratches and cuts your wash time in half. Once you do a decent paint correction, it will easily last for a year with occasional wax/sealant applications.
To maximize the lifetime of the resin, I only use the deionized (DI) water for the final rinse. I have a flat nozzle connected to the CR Spotless and jet the final rinse like I'm painting the vehicle - top to bottom. Another key to maximizing the resin life is that I have a whole house water softener. I have well water that is extremely high in calcium and magnesium. The water softener removes these ions and replaces them with sodium. The CR Spotless then removes the sodium.
Two other points:
1. The folks at CR Spotless are FANTASTIC when it comes to customer service. I needed a replacement part and they sent it free of charge and even shipped it next-day air!!:notworthy:
2. The best price that I've ever seen for the unit is from Costco. It's a bit pricey, but once you use it, you will never go back to drying any other way. Look at it as an investment like your polisher.