I've noticed that it depends a lot on water source and what they're eating. If they have good water and food, they will grow longer. Where I live, we've been in a drought for a few years and the food sources kinda suck, so they are done growing pretty much in late July.
My late July early August statement was just going based on some posts by Eli where he is posting scouting pictures and says that there maybe little more growth.
I think when it is all said and done any additional growth is offset by the bases drying and shrinking up a bit so the net score probably doesn't change much at all.
Other things I've read say they don't actually quit growing until the rut starts.
The one article that I read that is for sure correct is that they are done growing when you kill them but even then there was some speculation that some folks seem to be able to get them to grow a couple more inches by doing crazy things to them during the drying process.
I've only killed 3 pronghorn bucks in my life that I remember though and never pre-scouted any of them in the summer so take whatever I say with a grain of salt.
If they "grow" any at all past mid June, it's only on the bottom 1/2" of the horn. For any "scouting" purposes, what you see in mid June is going to look almost exactly like what you see any time later. Even Eli isn't going to notice the change from July-November when they shed or are shot. You can start scouting lopes in late May if you're looking for a big one, they'll certainly be big by then.