It's about a 40-45-50 minute drive from most parts of "downtown" Boston. For example, I entered the address from BU and it gave me a 41 minute trek to the stadium, which is oddly located in the suburbs near residential homes (Foxborough).
EDIT: There was a youtube vid showing the Pats buses driving through the residential neighborhood and arriving at the stadium, which really put things in perspective for someone like myself who has not had the opportunity to see it in person. Unfortunately, I cannot find the video, but someone else may know what I'm referring to.