Having a deadline just ensures that whatever I'm doing will get done, but it's usually very last minute. For instance, this past semester, I had about 5 weeks to write this 5-7 page research paper on George Orwell. It was the easiest paper I've done in a while, but I didn't start it, including the research, until about 7 pm the night before it was due, and I got a perfect score on it. All a deadline does to me is increase my stress, because I know I have to finish something; it's just, how much can I slack off before it needs to be done?
But if I don't have a deadline, then I won't get shit done. My high school career was like that; we could hand in assignments as late as the last day of school, so I usually wouldn't do the work until the absolute last minute.