We already know that Supernatural will be back next fall for a 4th season. However, what about after that?
Creator Eric Kripke has stated that he sees the show as running for only one more season after that:
"But we sort of have this five-year map. And Jared and Jensen like the idea of a five-year map because we work them so hard. It's possible, but I don't know. But a lot of these issues are beyond my control at the corporate level with ratings, etc. But my personal feeling is that I still want to go out in year five. It's better to go out strong and on top than to be 8 or 9 years into a piece of sh*t. You want conclusions - you want to end it. It's an epic story and epic stories deserve a good ending."
So...probably only 2 more seasons of Supernatural to look forward to.
I, by the way, agree with the creator on this one. I think it's better for a show to go out on a creative high as opposed to fizzling out over time (*cough* Smallville *cough*).