How often do you let your teenagers girlfriend/boyfriend round in a week?

[Cantstopworryingaboutit]

My 15 year old ds has pretty much moved his girlfriend in, she has been round loads in the holidays.

Now they are back to school, I'm wondering what is fair? I don't want her round every night, as he is failing all his subjects as it is, but equally I want him to be happy

My dd sees her boyfriend at school during the week but they don't see each other after school. They spend a lot of the weekend together

My son and his gf don't go to the same school unfortunately, as that would make it easier!