It really depends on how busy the car is and how much the OBS feels like hustling. It's about half an hour to PVD. I have been on trains where they have not even taken orders for food until PVD, but then on other trains they have had everyone's food to them by PVD.
I think the most important thing - if you want food - is to just mention it when you board. If you get a redcap you can often board 20 minutes prior to departure and they generally do drink service upon boarding. If you mention it ot them at that time, you might even be able to order your food before you even leave BOS.