It depends on how many ducks are using it, the weather and how fast slime grows. I have a tiny pond in my duck run for 10 ducks that I change every day. A little bigger pond in my yard that they use for maybe only half an hour or hour or hour a day, and I change that every week in the winter, and every few days in the summer.
BTW, big ducks do best with really shallow ramps, nothing steep, so you might need to make it kind of long.
BTW, big ducks do best with really shallow ramps, nothing steep, so you might need to make it kind of long.