Matt Weiner is reported as saying that the show, or more specifically, he, would not go for longer than 6 seasons.
The statement can be found here
A response can be found here
What do you think?
Personally, I don't believe that Mad Men has shown any signs of wearing out its welcome. I believe it should/could most certainly continue for another 3 seasons. However, in my experience, the cracks in most long-running shows start to appear around the season 4/5 mark. So I guess time will tell if Mad Men goes past season 6.
Also, I hope that AMC doesn't decide to continue Mad Men without Weiner in control. I remember that it took them a while to hammer out a contract for him to come back for season 3 (or was it season 2?) and I was holding my breath through that.