Time really isn't a factor. It's a three prong test:
1. Register a declaration of intention to marry with the county clerk (this has to be done together; one person cannot do this and sign for the other person).
2. Live together (have to show proof such as utility bills or a lease agreement in both names, using the same last name).
3. Mutually present yourselves to other's as husband and wife (simply saying you are married isn't enough proof. Legal proof is showing a joint income tax return, joint banking account, or joint lease agreement/mortgage statement; all listing both names, using the same last name).
You have to be able to prove all three things. If you stop living together, either person has 2 years to begin a legal proceeding to determine if an informal marriage existed. If one did, you have to get a divorce. If the 3-prong test cannot be proven, or if the 2 year statue of limitations expires, the courts say that no informal marriage every existed, and the people do not have to get a divorce.
It's pretty easy to understand, but extremely hard to prove. Most people don't know about the 3-prong test, so when things go bad and a relationship ends, they are really mad when they find out that they aren't considered informally married in Texas.
