My garage is 2 stories, 30 x 50ft and all reinforced concrete as I am in a hurricane zone. We built it as a hurricane refuge in addition to being a garage. I have overhead fans that blow toward my workbench and another fan directed out a window. When I shoot, the over head fan directs all the fumes toward the bench and the second fan takes the exhaust out the side window. The wall section the bench is on is about 12 foot wide and the trap is centered on the bench. I am not concerned so much about missing the trap as the window is about 5 feet from the trap, but rather that a friend in handling a firearm will have a negligent discharge in the direction of the large window, which is on the same wall as the workbench, but on the other side of the building. I try to think about all the possible safety contingencies.
I have been checking on ballistic panels and it looks like a 4 x9 ft panel that is rated for a 357 is about $400, so that may be an option. It's a lot of money, but cheap if it is ever needed.
So what you're looking for is protection of your window from a direct shot, then. That's much more involved and will require something rated for the highest powered round you will allow in your garage.
Using a ballistics panel of some kind would work, but you need to keep in mind what they're legally rated for in terms of long term use. What I mean is that a particular panel may be rated for a certain round at a certain velocity and maybe for a certain number of hits. However, that may differ from what they recommend in terms of maintenance. In other words, if a panel is struck by a round and you do nothing to repair/replace the damaged component, then in terms of liability and warranty you may not be covered for a subsequent hit by another round 6 months down the road.
Certainly hso's recommendation of 3/4 inch AR500 will work. However, at 31 pounds per square foot, that might present a few challenges.
Cost for some materials, like bulletproof polycarbonate, might give you some sticker shock.
http://www.eplastics.com/Lexan_Bullet_Resistant_Polycarbonate_Sheet
Bullet resistant fiberglass panels are probably the most cost effective solution for you. Armorcore makes some, probably around $400 for a panel, give or take. You'll have to do some more research on this yourself.
Alternatively, you could make your own bullet resistant panels. It shouldn't be all that difficult and you can test what you make for reliability and durability. Laminating some combination of sheet steel, wood, polycarbonate, and fiberglass, for example, should build up a servicable panel.