This is something thats bugging me lately too tbh. 
Always here, spends about 1-2 nights a week at her own place up road, they sit in living room mostly, make a mess etc.  Tbh i make a mess aswell but thers a massive difference when its just me living here and them 2 around, cos ill clean up after myself, but will not after them.
my privacy feels invaded when they sit infront of tv constantly, kitchen+bathroom is a walk through the living room. pees me right off tbh, iv mentioned it a few times but they just cant understand the problem, had enough now, if its not changed when hes moved back in after xmas ill have to have a proper word, not fair having her here 4-5 nights at all on me.
'get a girlfriened'   -   why should the OP have to be unhappy 

  some people dont want to spend all their time with their gfs tbh, and when you live with mates you wanna do mate things, without having their other half about?
My mrs comes round and spends the night once a week at most(we live away from each other so its weekend timetable for us, but one or the other often has something on fri/sat).  Actually thats a lie, at most we see each other once a week, and i go to hers, so thats once a fortnight on average, but even then, its less cos we go 2-3 weeks without seein each other sometimes. 
Have a strong word with him, electricity racks up, tbh have a word with the fact hes becoming sad more than anything.  22 spending all that time with a 17year old?  unhealthy.