I couldn't tell you exactly how fast it clears, but from my testing it looks like it maxes out at 50mj/t, anything above that doesn't make it go any faster. If I had to guess it was clearing 5 or 6 blocks per second when it was going the fastest.
Also if you didn't know you can use landmarks to section off an up-to 64x64 area for the quarry to dig, you'll have to move it far less often.
Grumpy's video where he shows how to setup a quarry using solar energy goes over the landmarks:
http://www.youtube.com/watch?feature=player_detailpage&v=6oo1UfNnOc8#t=475s (obviously this is out-dated information when it comes to the max speed of the quarry, you need 5 electrical engines each with 2x output boost II to equal the new maximum of 50mj/t)
