What is important is the time passing since the live-in relationship began, not just the bank account. If you think about things from INZ's point of view, they aren't allowed by government to give visas to people whose relationship is going to break up any minute, and this regulation is gambling on how long it takes for people to be sure they're well suited for the time being. They can't look into people's feelings, so it's all down to something actually provable, in this case, living under the same roof. Also, you know you're decent people, but the regulations are formed knowing that partner-sponsored visas are the ones most open to fraud, with people paying citizens and residents to lie to get them a visa, so all applications are VERY carefully scrutinized.
So what have you got to show the early time when you first lived together before you opened the bank account?