I did that ride on the early end of morning rush to catch my Thanksgiving train. We left our house at 6:50, fifteen minute walk to Davis, wait for train, take train, and were in the main concourse at South Station with a comfortable but not excessive margin before our 7:15 train. (So I'd guess the on-the-T portion was about 25 minutes, but it's not like I can precisely remember the duration of T rides at 7am. ;)