If your cat isn’t drinking all of the water in their fountain and you’re topping it up before it’s empty, you should completely empty it and change it every 1-2 weeks at least. It will need to be more frequent if you have multiple cats. This helps to ensure that the water is kept completely clean and free of dirt. A good way to tell is if you see the water start to bubble or show foam. White bubbles and foam are usually a sign that the water has been contaminated with dissolved cat food or saliva, and while not especially harmful it does mean the water isn’t as clean as you would want it to be.