The Joker in Dark Knight executed some very careful and involved plans. For example, your quote is taken from his conversation with Dent, which was the final step of his plan to drive Dent crazy. Hell, remember the heist that the film starts with? Does it look unplanned?
This is totally off topic, but the fact that The Joker eschews planning could also be part of the theme of him being an "unstoppable force" rather than just a man. He symbolizes man's darker nature and tendencies.
Having a Plan and thinking more than 24 hours into the future don't neccesarily have to be the same thing. I took his use of "plan" in the grander sense. Why did he plot to drive Dent crazy? Who knows, he has no Plan, he just did it.
