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.