My girlfriend has now got a two year visa to come to the U.K.

I know she is only allowed a maximum of 180 days a year, my question is…. When does it start, 1st January each year or does she have to stay a certain amount of time out of the U.K. before coming back.

So after say six months in the U.K. she goes back to Thailand in November can she start again for the next 6 months on the 2 January.

Thank you.