Each case will be different. So there is no easy answer but, basically, you will need to demonstrate to a visa officer that you have been living together in a genuine relationship for 12 months or more. You can do this by providing copies of documents that show you have lived together at the same address for example joint leases or property records, joint bank accounts, utility bills or letters from friends or family, or anything else that you think can show that you have been living together in a partnership. Most likely, you will need a combination of documents, not just one.