I have changed mine twice since having my car (with 52,xxx miles). I have it set up for around 20k miles now to go along with servicing my transmission. Its a rather simple task of removing the two bolts/plugs in it (to drain), then putting the lower bolt/plug in to fill. Its just a bit tricky to fill. Like Paul said its a good idea to keep fresh fluids in there to guarantee a proper working and long(er) lasting part to your car.
"2007 WRX- PPG'd, 20G- Tim Bailey tuned
Stage 2 2005 WRX