Unless you have a leak in your pool, I would guess that you're going to use 100 - 150 lbs for the year....but then again, it would depend on backwashing frequency (which you won't need to do with a cartridge filter), bathing suit drag out, and splash out. Oh, and leaks, and rain water overflow/draining will also lose water and salt.
You're not going to loose salt in the process of generating chlorine. Evaporation does not lose salt.
Finally, depending on the recommended salt level for the unit you're interested in, you may need to add alot more than 100 - 150 lbs/yr. Some imported salt chlorine generators need 4000 - 6000 ppm of salt to operate properly. For those, you will need proportionally more salt.