If yuu are going at any of the rush hours ( there are several both in the morning and at night, y)ou would be better off getting a monthly rail pass be able to have a coffee read the paper, work etc. The commute into Boston has been getting ugly lately and with the Callahan closing soon will be even worse.
Bottom line depends on where you are working in Boston, if you have free parking, do you need your vehicle during the day etc. but I would say a minimum of 1 hour, up to 3+ if you get caught in a snow storm. ( been there, done that, don;t want to do it again)