That's kind of an open question. We have made the trip several times, sometimes driving straight through, so we stopped at Flying J's, Rest stops, etc and make it in 3 to 4 days from Nova Scotia. Sometimes we take 2 to 3 weeks, sauntering troughout the North East, which has sooooo many beautiful areas to explore. If you are looking for metropolitan areas, I'd recommend a few days in Boston, agriculteral then a few days in Lancaster Co Pennsylvania, rustic then the White Mountains, old grand houses will take you to Newport RI. So, my best answer to you is to be flexible, take as much time as you can and see as much as you can.
As for "leaf peeping", it is usually around the Columbus Day week end, but it does vary depending on elevation, how far north or south you are, and on other variable such as the weather.