How often should adults deworm?

Hard to say: As a USA based site with mostly docs from the US this would never be considered a routine process. If worm infestation is common in your part of the world, it might be, but you would need to consult local health authorities in your country for that information.

Only when needed: Adults should only deworm when diagnosed with worms. Parasitic infections are relatively rare nowadays specifically in adults, but should be treated timely when diagnosed properly.