More like CYA goes down slowly as you replace water due to evaporation and as you drain off excess water from heavy rains. Or in our case when you replace water from Golden Retrievers carrying it out like giant sponges
Like salt and calcium, CYA does not leave your pool through evaporation. Unlike salt and calcium, it does breakdown a bit over time, but not particularly fast. As pointed out by 1829, salt and calcium and CYA do leave your pool due to splash out, but that rate of loss is determined by how much of your pool water you're losing that way. I replenish a small amount of CYA in my pool about once or twice a year. I test for it a little more often than that, but not weekly.
The expected drop due to degradation is 3-5 ppm monthly. Sometimes more in the peak season. Also for those of us who get 3 ft of off season rain, that can be a 50% or more water exchange with a mesh safety cover. I close around 80 and open up under 20 every spring between the two.