Depends...
It depends on what you are doing with the car.
If you go to the track you will obviously be doing it more often(or at least you should

). Brad Neff ("SubDriver") is a forum regular, a successful racer (SCCA T1), and he has had better results with his maintenance routine than most. I put a lot of weight into what he says and does. Since I go to the track for DE's and open lapping days, I follow his advice and change the fluid every three weekends (6 days of track time). By the way, I also get all my fluids from him. I like supporting his Amsoil business, which in turn supports his race team and this site. :z:
If you don't go to the track, it still depends on how hard you run your car on the street. I have the three-volume shop manual set at home, and I can see what it says. Someone will probably beat me to it, and post the info.
For moderate driving with an occasional spirited outing on the street, I would probably change the differential and tranny fluid every 20K miles. I know some guys who do it as often as every 5K, which is a bit much, especially if you are using a good synthetic.
Hope this helps! :thumb: