When to clean filter?

I've got an Oase Filtoclear 800 for my small (400 gallon) pond. It has
a lever to backwash the filter and send the dirty water out a
different tube. I'm to turn the lever, then pull on a handle that
apparently compresses the foam elements in the filter, helping to
clean the filter.

The question: When should I do this, and how often? Isn't the filter
supposed to build up beneficial bacteria? Won't the backwashing
process just wash away this good stuff? Also, do I need to do a more
thorough cleaning of the filter than this?

Many thanks.