Standard falling accelleration under normal circumsances is 10 meters/second squared. (10m/s^2)
After 1 second, the rock has fallen 10 meters, and accelerated to 20 meters/second
At 2 seconds, it has fallen 30 meters and accelerated to 30 meters/second
At 3 seconds it has fallen 60 meters and squashed whatever was trying to move out from under it.
Terminal velocity for a rock is much greater than that for a feather for instance, and would be based on a few variables, but that sort of acceleration would continue at least out to 100+meters/second, especially for a rock of normal density. The maximum damage from a fall rules is based on terminal velocity (if they put that much thought into the rule) of a person or creature, not of a rock. So the 6-7 seconds thought would have to be from a VERY high place.
I also imagine it would be a very high spot check to notice where the Monk disappeared to. Even if you saw it, you would only get a Reflex save in my game, not be able to move out of the way of something going that fast automatically.
So, touch attack on the square of the target by the monk, spot check by the target to see the attack, and then possible reflex save by the target to avoid getting squashed is how I would rule it.