A: As often as the airbrush needs it, but not so often as to get lube into your paint.
It’s not really a question of schedule like changing your car oil every 3000 miles. There’s only two places on an airbrush that require lube and an optional set of locations.
The piece, deep inside the airbrush, needs just a little lube to let the needle pass through it without problems.
Notes: Teflon Needle Packing O-Rings don’t need lube, but the action still tends to feel better if you do.
This o-ring seals the air system from the trigger housing. The air piston, or dangly piece underneath the trigger depending on your model, needs just a little lube to let the trigger spring up and down nicely.
Screw threads don’t NEED lube. In fact, I prefer beeswax since it’s natural, but lube works too. It works as a temporary, non-drying sealant to keep various kinds of leaks from happening. Not to mention it ‘feels’ better to screw things together that have a little lube between them.