Man, I had a blast scouting last year for Pronghorn! I think I started going pretty hard in about May, as we were bear baiting on the two mountains on either side of the unit I planned on hunting. I carried that into the following months with whenever I could make it out there. Even the trips where I didn't find anything new as far as herds, I still learned the roads, public/private land and had a great overall concensus of the area well in advance of my hunt.
You can never scout too much... Just be aware, that where they are right now might not be where they are in Aug/Sep, but they will be in the same general area.
Good luck on your hunt. Are you doing Spot and Stalk or a waterhole? Hunting these guys is a blast, and if your anything like the rest of us, you'll fail on several stalks, They are a very humbling animal!! Especially with a bow.