As the others have mentioned, at that time of day, it is like a lottery. There seem to be a number of international flights that come in around that time. If the flight is early, you might walk right through. I've never had it take more than a half an hour (once you get into the building ... remember IAD has shuttles that come up to the plane, so you may have to wait a bit when deplaning).

I'd allow 90 minutes to 2 hours to travel up to Baltimore. It's a nightmare that time of day. If you can get on the BWI flight, it is far more convenient for Baltimore (not to mention extremely fast to clear immigration/customs).