I literally just made it lol... But if we estimate by multiplying me amount of miles by 1.5 based on my experience riding the Lowell line to visit UMass Lowell I'm guessing the end-to-end time would be somewhere around 290 minutes, maybe 280 because this line would probably be electrified and be more efficient.

All in all, you could expect somewhere around 4 to 5 hours to get around the whole thing, but it's not meant to take people all the way from Brockton to Gloucester; it's meant to carry people short distances that require stupidly long journeys into Boston and out of Boston again, such as Fitchburg to Worcester, or Foxboro to Brockton.