I use the cap from a gallon water jug. I just put in a pinch and change it at least once a week. I rinse and scrub the cap about once a month to make sure its clean.
I change mine when the girls walk thru the cap and flip it over...Dumping all my calcium all over then they walk thru it and make a mess for me....Uggghhhh..... Usually about once a month...
not too often, probably once a month or so. sometimes i see it looks gross like they walked through the water and then walked through the calcium making a mess so thats my cue.