You would probably need to prove 12 months living together...
When we applied for the visa, we were married, but only for a few months. Despite that, we had to prove that our relationship is genuine, going back ONE year! Even so we could prove that we had lived together for over 15 months (council tax bill, change of current account address etc.), still we were required to provide a statutory declaration as well!
We have filed the visa Application in May of this year, so the info is recent. Also, it very much depends, despite what others say on your Case Officer.