It depends on the angle of the traveling rock as well as how much it slowed due to wind resistance before it hit you. Velocity of an object can be described in vectors, so if you know you remember you trig and now the velocity and direction of which the rock was traveling, you can figure out how fast it hit you at.
If you know the mass of the rock, you can figure out how much force it exerted on you too.
Glad you are ok though! I've had a decent size rock hit my visor that made a rather large noise...noise = energy so...ouch if it hit my eye!