You can look up the minimum connection time required for various airports, including IAD, in the UA timetable (available on their website as a PDF file). If you schedule a connection time that is too short, I doubt they will feel obligated to do much for you if you miss the flight to BRU. Whether they will feel such obligation if you allowed enough time for the connection, and the BOS-IAD flight was massively delayed, is an open question. Good business practice would be to try to accomodate you.