depending on ur bio load.. will determine when its time.
every 3 months sounds good too me for biological media to be cleaned. only 1/3 of it should be cleaned, i can not stress this enough!!!.
mechanical media could be almost every 2 days for some........ or once a month for most people. its good too look also at the media see how gunked up it is, or if u water turns green or milky or fish start to die thats a sign.