I guess it's probably different if you get into a relationship while still in high school or an established adult with your own place to live, but my partner and I were pretty much living together within a month. We're both in college and I basically moved into his dorm room. My (university assigned) roommate was a mess and I was tired of dealing with her coming in at 3 AM high out of her mind and deciding to bake cookies and his roommate was his sister, who spent 98.5% of nights at her boyfriend's apartment. Since he had the space and I wasn't getting enough sleep in my assigned room, the simplest solution was for me to start staying there. My assigned room became more of a place to store my things and I generally only went back to get clean clothes every few days. This past August we moved into an off campus apartment together. We got a kitten in September and we're living together this summer and next year too, and hopefully for years to come.

For me, at least 6-12, just so you've probably spent a few continuous days in the house together. It does vary a lot by couple, so go with what feels comfortable for you.

I would say once the relationship gets "comfortable". Can you take a dump when the other person is in the other room? Do you feel the need to hide all of your less-than-glamorous beauty treatments? (Plucking, shaving, wrinkle cream?) It'll be a mess if you feel that you need to hide things like this and you move in together. If you can handle all of this, and manage to not spend every waking moment having sex, you're probably good to move in, but it'd be a good idea to have a spare bedroom, incase things don't work out and you're not ready to move out the second things go south. (Or, you know, for guests.)

Depends on the relationship.