any time after 6 months of dating . Yes live together before marriage, I firmly believe it's a good thing to live with your "potental" to see just what they are like when they let their hair down and relax in a home sitation.
I lived with both my wives before I married them.
i know people who moved together after knowing each other for only 4 months and they argue a lot. But it doesn't mean you need to wait years and years. It depend on how close you feel to each other.
If he waited on me to marry we would never live together