James Hibberd has the Mad Men master plan: five seasons covering a period of ten years, 1960-1969/1970. We'll clearly be working off a Sopranos model, which means we shouldn't necessarily expect each new season to pick up where the last left off. Via AICN, which summarizes thusly:
* The series is designed to run five seasons.
* The show will skip long periods of time between seasons. (Season one concluded in late 1960; season two, premiering July 27, picks up in early 1962.)
* The series will take the employees of Sterling Cooper at least to 1969. Perhaps even 1970, since I'm told Weiner said "10 years."