I had once asked a similar question to the MBTA and got this response back:
If you park at an MBTA facility for more than seven consecutive days, your vehicle will be considered abandoned and it will be subject to MBTA Police ticketing and towing. If you plan on utilizing long-term parking please tell the attendant you plan to park for an extended period of time and give the attendant your license plate number.
If you are using the transportation system, overnight parking is allowed at MBTA Garages for up to seven days. Any vehicle parked at an MBTA facility for more than seven consecutive days would be considered an abandoned vehicle and would be subjected to MBTA Police ticketing or towing.