Before I made my own, I ran my Hagen for about 3 weeks, poured about an inch off the top, added new warm/hot water and let it go about anther week or so. I would say once a month.
I did something like Purrbox said, but I replaced the bottle completly with 2x 2 L soda bottles. I new replace one every every other 2 week. and I've not had a problem since.
I use a beer yeast from a local brewer. Works great and is cheaper then the refils for the hagen.
GOod luck to you.