The depth of the foundation should be beep enough to properly embed the anchor bolts. This would vary based on the size of the equipment. First, consider the mass required for the foundation. Next, consider the 30 degree rule for a stable footprint.
The 30-degree rule is a best practice. If you drop a plumb line down from the center of the rotation shaft and rotate this line 30-degrees in both directions, the line should go through the bottom of the foundation and not the side. This will ensure stability of the foundation.
The foundation should be of sufficient depth to prevent damage to the piping or machinery due to frost penetration. This depth can vary with geography. Also, consider the 30 degree rule and the required mass.
