I don't think the question is easy to answer :-) There are several factors involved, e.g. how many pets use the drinking fountain.

Our experience so far with a short-haired cat:
- 1x per month there is a major cleaning: Replace the filter and clean the drinking fountain properly, as mucus deposits build up (instructions can be found on YouTube or via the manufacturer's website)
- Water is topped up if necessary but not really replaced completely.
- Depending on how dirty it is, the cat hair on the surface must be removed or the filter must be rinsed with water.

On the whole, however, it is a simplification for us compared to replacing a water trough every day.

We change the water every day and the filter every month

Once a week is enough.