Is there a law about how many nights a person can stay over before they're classed as living there?

I'm worried. My flatmate's boyfriend has been at ours nearly every night since November (barring our Christmas break when both of us were at our family homes). We're in a two person flat in Scotland, and her boyfriend lives ten minutes away. There is no HMO on the flat, although I've seen conflicting reports as to when that's needed. Are we actually breaking the law with him being here?