Sorry if this has been asked before, I couldn't really think of a suitable search query to find it and couldn't find mention in the threads I looked at either. How long can EU nationals with B permits be out of the country without having to reapply when coming back to Switzerland?

As far as I know you need to ask for an extention _before_ you leave. Ask your gemeide/kreisbüro, they know.

6 months.

It is written on the last page of your permit.