Checking out the attitude mode when stationary may not give you the sort of drift errors you are looking for because you will have no accelerations. Without these, the errors will not be anywhere near the same.
But the main reason for the 'north' direction to change is not system errors but simply the rotation of the earth, and that will be the same. So you should see roughly the same effect in the air as on the ground, and the formula has already been given: 360/24 * sin(Lat). Just like a perfect, i.e. precession free, directional gyro.