30 minutes was a wild guess . . .
If I were making this trip, I'd get on 128 directly, and avoid the central artery altogether.
A friend of mine used to live in Quincy - I have to say, I think Quincy has the worst traffic in the entire Boston area. I hated driving in that town. Then she moved to Weymouth, so I had to drive all the way through Quincy to visit.