How often should air spring forks be reinflated (topped up)?
For one, it depends on how sensitive you are to changes in suspension performance. You can’t really measure for any drop (your shock pump will initially steal some of the air chamber’s air in order to equalize the pressure), so you’d have to go by feel.
From my experience, leakage has been negligible, even after a few months. Topping the air up isn’t hard anyways, so it’s not a big issue. Just screw the pump on, get back to your original pressure, equalize the positive and negative air chambers (if applicable), and you’re good to go. If your fork/shock leaks faster than that, you probably need to do a service.