"50 to 100 yards" according to Boston Police Commissioner Ed Davis in a news conference. A lot of newspapers printed this as though it might be true. Canadian media settled on 100 meters as a good-enough approximation. I was pleasantly surprised that Wikipedia (when I checked earlier today) had the blasts occurring "within 550 feet" of each other — somewhat closer to the truth.
The blast locations are no secret: There are plenty of photographs. The first happened in front of Lens Crafters at 699 Boylston; the second, in front of Forum at 755 Boylston. Some folk tried to place the first blast in front of Marathon Sports, next-door to Lens Crafters, but the damage done to the Lens Crafters facade speaks for itself.
So we know each location within a meter or two. Using Google street view to familiarize oneself with the street and building appearances, one can — in Google Earth — situate correctly both locations using the ruler tool: 183 meters, give or take.