There isn't really a "standard" but a .010" on the bottom (pinion side) and generally a fiber washer, and anywhere from .010-.025" of shimming up top is real common.
You normally want the comm in the middle of the magnetic field.
Assemble motor without shims. Hold motor horizontal and spin arm.
Grab the arm and pull, make a mental note of how much it moved "out" of the can .. this is your end play. Add approx the amount of shim, you want just a "tick" of end play ... .005-.010"
Repeat the process but push the arm into the hood, this will give you a guideline for how much to shim the top of the arm. Overall the arm should not move back and forth more than .010" but it's better to have it move some rather than not at all.