Good recs are important, as are rockin' decks, but without clean-sounding needles the best DJ in the world is still going to sound like crap. This article isn't going to cover the kinds of needles you need we'll save that for another day. Even Steven First off, needles are like tires. You don't ever buy just one tire, you buy enough for your entire car. I can't stress this enough.
Manufacturers recommend about every six months.
However, manufacturers also want you to spend lots of money. I just went a whole year without changing, and when I finally did I noticed a big difference. So I'd say around nine months. Of course, the more you spin, the sooner you should change your diamonds. That way you don't have to replace the entire cartridge.