Chemicals you add to your pool usually mix rather quickly. Usually within an hour, your entire pool will have some of the chemical you've added to it. I did a test using some colored dye and found that when the pump was running and when I poured it over the return jet in the deep end, it spread fairly evenly throughout the pool within a half hour (most of it within 15 minutes).

Now there are some websites that I've seen that say you have to wait longer before measuring pH because it takes a while to stabilize, but I've never seen any chemistry justification for that. All of the reactions that I'm aware of in the pool are very fast (except for chlorine breakpoint). Only the slow pH rise due to outgassing of carbon dioxide is very slow and that will never stabilize since you always keep your pool intentionally out of balance in order to maintain a TA level (with a pH near 7.5).

So my opinion is that you can test your pool chemistry an hour after you've added chemicals and you are likely to get a decent result -- so long as you have your pump running the entire time.

Richard