Depends on what media you're using, if you have the finer grained ARM stuff I'd replace it all as it gets pretty slushy after awhile. Usually I'd let the reactor run to within 3-4" of the bottom, then replace it.
Now I'm using the larger grained stuff, its almost like 1/2-3/4" chunks of coral, I just top it off when it gets down about half way or sometimes if its looking pretty gunky I just empty the reactor and give it a rinse, clean out the reactor and put it back in with new mixed in.
I always liked the finer stuff better though.
