Depends.

Sometimes I plan farther ahead, sometimes I keep the motion of what's important in the short-term. Other times I do a "touch-and-go" approach and see what sticks. I’m of the opinion that you should have an idea of how a story starts, an endgame, and your foundation. Planning too far ahead cripples your project as you don’t know how your ideas will change or your players will affect the motion of the story in a different direction.