Depending on your pool's circulation, the pH change could be reflected in 20-30 minutes though I'd wait an hour to be sure. Most chemicals mix thoroughly in the pool rather quickly and the reactions in the chemical buffer system are very fast (in human terms). The only exception would be "thick" viscous products like PolyQuat 60. I've done tests in my pool that show fairly thorough mixing in 20 minutes when pouring chemicals over a return flow at the deep end, but I've got two floor drains and a skimmer opposite the return at the deep end and two additional returns in the shallow end. For above-ground pools that typically do not have a floor drain, the mixing will take a lot longer though you can speed things up by lightly brushing with a pole or otherwise stirring up the water.
Richard