Every 16K miles. I did it at almost 15K (14924) and all eight valves were at or slightly below the minimum clearance. So I am dropping everything two shim sizes (0.05 mm) in hopes that they will still be in spec at the next inspection.
This isn't meant to sound harsh, but, I really want to only install OEM shims.
They are Hot Cams shims. Been around forever; the quality is fine. Yes, some rarities are slightly different machined thicknesses than reported, but that's why you ALWAYS mic shims before you install, regardless as to where you got them. Just an FYI, 2 of my factory shims that came in the bike were 0.01" off at my valve adjustment....so make sure to put the micrometer to them.