Normally, when I have a friend, and I really like them as a friend, there always over at my place ( my friends and siblings friends always want to chill at our place instead of theirs, nice atmosphore (sp?) I guess? :\ ), and usely, if they blend in with the crowed ( my family, meaning: their not loud, annoying, whiny or needy ) they can stay with us as long as they like, really.
In the summer, I've had friends stay over for like, three days at the most, and then, come back like, two days later
So, before you ever want to really hang out with your Best Friend for about, three months, its best to live with them every other weekend of so to make sure you 'really' want to do that
Or hang out as much as possible xD
So in short, live with them first
~~Katura
P.S. Normally I would know them for at LEAST a year for them to come over
that much anyway.