This can vary depending on usage and water quality, but the best indication is when the water flow becomes noticeably less than normal. This reduction in flow is due to contaminants blocking the water pathway and proves that the ceramic element is successfully filtering out foreign bodies. Another consideration is the colour. When new, the colour of the ceramic water filter is white; any dis-coloration during use is due to contaminants being filtered out. For optimum performance, clean as explained in the question below.