once per season, i check my sand for it's flowability..in other words..can i put my arm in and move sand around easily throughout the filter? what i don't want to do is wait for 3 or 4 years, check it then and find out it's hardened up. i don't think you can do much at that point except buy a new filter (at least, that's what the pool store says and i'm inclined to believe them as i've seen filter sand that is in hard chunks, bottom half is hard, etc. my sand is at least 5 seasons old and likely longer so i ordered 2 bags of zeobrite and am going to change my sand out to that this fall..then in terms of how long there has been a material in the filter, i know exactly what i'm dealing with. (my problem is that the prior home owners were terrible at maintenance in general around the home..even little things like tightening a door handle, so it maybe 5 seasons old..or it could be 15) my sand is still very porous at a check a few weeks ago so i would like to believe it's no more than 5 years old..also, i have no idea how long it takes sand to go from porous to hard in a filter, which would be nice to know.