Over the last several weeks, my pool water has been a little dull. It just didn't have that crisp, clear look as before, even with "perfect" chemistry and chlorine. I attributed the dullness to all the rain we've had. My sand filter is 33 months old with original sand. Tuesday night (night before last) I changed out the sand. The next day (yesterday), the water looks remarkably better. Another thing, the new sand seems finer than the old sand. Anyway, the question of when to change sand has been answered, at least for me.
As a side note, after I changed the sand, I put a single trichlor puck in the floatie since my cya is too low. When I told my wife how much better the water looked, she thought it was because of the puck! Lol, after 4 months of BBB she's still not convinced