Common Law Marriages and Income Tax Filing
Amazingly, Colorado has no law that states what constitutes a common law marriage.
In practice however, a couple can file a joint return as Colorado residents as long as they meet these five widely-held tests: 1) they are not already married to someone else, 2) they present themselves as being husband and wife, 3) they consent to the common law marriage, 4) they cohabitate, and 5) they have the reputation in the community as being married. Further, a signed affidavit can be presented when proof of common law marriage is required (e.g., for insurance).
As mentioned above, cohabitation is required but no specific duration is needed. The most critical item that can negate common law marriage marital status is not regularly introducing or referring to each other as “my wife…” or “my husband…”
Filing joint tax returns is widely regarded as the most important signifier of a common law marriage as the couple is representing themselves to the government, under penalty of perjury, as being married.