Women's fashion suits might want to wait till it's 15 or so. You just need to wait long enough for it to mix, so it's 20 all over, instead of 10 in most places and 40 near the returns. If you add it via bleach poured around the pool . . . 15 minutes should be plenty.

With a CYA of 80, 20 ppm is not going to have ill effects on swimmers.