I bought it about two months ago. In my case the problem is not the fountain itself but...the cats! They hardly want to drink to it I am very disappointed as Peppito loves to drink fresh water from every tap he can find, but here... he is scared! And Jasmine sometimes try it, and sometimes prefers the classical water bowl. After some time they drink a bit more to it, because I sometimes remove the water bowl and they have no other choice. But I don't dare removing totally other sources of water and that they don't drink enough anymore

Ok, but this was not your question! In such a short period of time I did not have problems yet. I am using cheap spring water from bottles as the water where I live is extremely hard.
After three-four weeks I stopped it and emptied it because we were not at home for one week. When I came back I wanted to change the filter as recommended but except some hair it did look OK. So, I carefully washed it and replaced it again for two more weeks. It is probably time to change it now. May be Kim will give better advice. I am interested too!