In my experience, it doesn't matter whether you let it sit for 3 minutes or 5, or 8--the reading is going to be almost the same. CYA is such a subjective test and so many things can interfere with it--lighting, temperature, etc that your best bet is just to make sure that you are as consistent as you can be when you test, so that your results are as consistently true as they can be.

When I plan to test for CYA, I use water at room temp, I mix my water and reagent, shake for 30 seconds, and set it on the counter while I run all the other tests I'm gonna run. After that, I do the CYA test, in the same place in the kitchen, with the same lighting and blinds open, and facing the same way. Sounds silly, I know, but I have flourescent lighting above me and a picture window behind--if I test facing the window, or with the blinds closed instead of open, or with the overhead light off, it skews the result. As long as I test with the conditions as consistent as possible, then I know to trust any changes that my testing shows me.

Janet