Visiting periods

Hi all, and thanks in advance for any help.

I'm trying to clear something up, and I keep getting conflicting information. I'm.hoping someone here with experience has some light to shed.

Is there a minimum amount of time you have to be put of the UK for before re entering as a visitor?

If someone has done (almost) 6 months as a visitor (EU national, no visit visa required) do they have to be out of the UK for 6 months before they can re enter and visit again for up to 6 months?

Essentially, is it up to 180 days at a time, or up to 180 days in any 12 month period?

Thanks