again, I'm new, so get others' opinions! i've heard anywhere from every 6 months to 1 year.
they are simply pulled out and rinsed out. you dont want to get rid of the good bacteria, but you do want to wash out any particulate matter which accumulates.
that particulate matter is the problem with the bio-balls. as it decomposes, it creates ammonia.
i was once advised to use them for tank cycling and pull them out a handful at a time as the tank stabilizes and the bacteria developes in the live rock.