Most Helpful Guy
I don't think people should give themselves a set rule here. Instead, it's like with everything else in a relationship: when it feels right, it is right. This can happen after 3 months or 10 years. None of these options is better or worse. Some couples can live together immediately and it works great, others need or want to wait much longer.
I've always felt like the big test whether you fit together with your boyfriend/girlfriend is to go on a vacation for at least 2 weeks. Contrary to other times, during this vacation you really get to know your partner, including all his/her bad habits. You are together 24/7 and you have to perhaps deal with some stress. This is a great opportunity to find out how you can fight together. Can you fight well with each other (this includes making up soon after) or does it result in a huge, endless drama that lasts for the rest of the vacation? Is your partner able to communicate well or does he/she just swallow it up if they're angry at you?
If you're still happy and in love after such a vacation and everything has gone well, chances are you will also do well once you move in together.1
Most Helpful Girl
Years. Most research says it takes 2 years to actually get to know someone. Why tie your lives together before at least a year. Then when you move in together you spend the time hiding habits you don't want the other to know about. Not to mention the first year living together is generally the hardest. If you plan to get married more research says 1 or both of you expect living together to change after you've said "I do". Nothing is going to change you're already living together.2