How deep should piers be for a concrete block fence wall?
Concrete block walls don't have posts or piers, usually. They rest on a continuous footing. The depth of that footing can be found in established local code, and depends almost entirely on annual frost depth. 

If your wall has support beams of concrete or steel you could build on piers, but that's a much more expensive and complicated approach. However, the depth requirement for either is probably the same.