It should be changed every 3-6 months. If your filter looks very dark and dirty, it might be time to change it.

It depends on how you use it, but before it falls apart it's best to change it every year, especially for hygiene reasons!

The filter should be changed every 6-12 hours, as can be seen from the operating instructions.

Every 3-6 months
I also do it by feel from time to time