How much time should I allow if hitchhiking from Southeast Michigan to Boston, MA?

Personal experience: it's better to wait in Stockbridge until you can get a ride in to at least Newton where you can pick up the T. Even if being selective delays you. The police start getting more militant beyond Stockbridge. Plus competition on the exit ramps. Springfield or any other place closer in but beyond the T's reach is horrible.

Stockbridge is ok. If you want to reduce hitchhiking time, route yourself through Stockbridge and wait there until you get a ride all the way to somewhere near a T stop. Stockbridge in this context means the Turnpike rest stop or even inside the village itself.

A basic estimate for the time needed to get from Detroit to the greater Boston conurbation is about 18 - 22 hours on a good day.

My advice (for informational purposes since this is long outdated), is to do it by truck. I wrote a little about tractor-trailer hitching on hitchwiki. Generally, in the USA, I find that it is usual to wait 1-4 hours for a ride. This applies to on-ramps as well as truck stops.

Best-case scenario is it takes only two rides to get there (trucks are legally limited to 10-hour days, I think). If you get rides in an hour each, that makes 14 hours.

Worst-case, assuming you know how to hitch efficiently (have you done this before?) and don't get stuck, I would say two days in trucks. With cars, it could take more than a week.