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...
What about building a wood ramp, with some railings, and covering it with astro-turf/ or that cheap indoor outdoor carpet you can get at Home Depot? Then put a couple steps made of concrete blocks and pavers on the inside for them to step out on?