Normally, you don't have to change the sand in your filter. I must disagree with Tenax. Unless you are having a problem with your filter, don't maintain it, or just switched from bacquacil, you should not have to change it.
OTOH, filter sand is cheap (unless your pool store is ripping you off) so, if you are careful not to damage your laterals, it's not expensive to do.
But I'm 4 years into my sand without a change and my water is crystal, crystal clear. Sand filters better as it gets older. I use a little DE everytime I backwash, and a skimmer sock.
But mainly, I keep my water clear, clean and balanced.

it also showed me that my prior tecnique for inspecting is not sufficient. gray stuff aside, moving my hand through the sand was not sufficient to feel the amount of clumping, in spite of using filter cleaner according to instructions all of 3 weeks ago. (clumps were breakable when vacuuming them out of the filter, but certainly not soft) 
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