We recommend changing the sand every five years. While we have seen filters go 20 years or more without changing the sand and still do the job, they are not as efficient as they should be.
Filter sand has been ground to a size of .45 to .55 mm in diameter and is very rough when new. This roughness is what makes the sand efficient at filtering out the particles of dirt in your water. As this roughness is smoothed out - as stones in a stream wear smooth over time - your filter's efficiency goes down. This means that your system has to run more frequently to accomplish the same task.