No expiry in law. The Maine Human Rights Act contains no validity period, and the Maine Human Rights Commission does not register or date assistance animal documentation.
Twelve months in practice. Maine housing providers, like those everywhere else, treat older documentation as evidence of a past situation rather than a current one.
Maine licensure is the check that catches people. Seasonal and cross-border moves are common here, and a clinician licensed in New Hampshire or Massachusetts is not licensed in Maine. Sort that out before your next lease rather than after a refusal.