Velogirl has a point about time, but the intensity of my mtb rides is far more than my road rides due to steepness, technical terrain, entire body in use (including more gray matter), etc. I've always used a WAG of 1 mtb mile = 2 road miles which also seems to be about the equivalent ratio time wise.

Two hours on technical singletrack is far harder than almost any two hour road ride I've done unless I've purposely gone out for repeats on a steep hill.

I do use time as my primary measurement when I'm using mtb rides as training for a road century, and then factor in the other stuff, but in general 1 mtb = 2 road.