Starting in November 2014, you can look back three years before to recover unpaid overtime. This expands the previous Massachusetts overtime statute of limitations by one year. (It was two years, now it is three). Lawyers are now debating whether the change is retroactive or not, and the courts will weigh in on that question in the future.
As always with statutes of limitations, you only lock in the time at the beginning of the period once you file a case in a court. (Although, also starting this month, the filing of a complaint with the AG tolls the statute until she issues a private right of action letter.)
A brief word about the federal statute of limitation for overtime cases: The look-back period is two years, unless you can prove that the employer was “willful” in its failure to pay lawful overtime. If so, the limitations period is also three years.