How often do you change the aquarium gravel in your tumbler?

I've discovered that I can get my coins even cleaner if I soak them in hot water and comet in a glass jar for a week or so, shaking them from time to time. Then change the water/comet and repeat. Then put them into the tumbler for a few hours. I also noticed that alot of silver clad will wear down too much and pit, or the "copper color" will stay on them. Some silver will come out spotless after tumbling. I figured one batch of gravel would last quite a long time, but also wondered if they "lost" their abrasive characteristics?