How do you set deadline for a task?

I mean obviously you cannot endlessly continue a task at hand. Either your manager or your client would definitely ask you when the product would be ready. We divide the work into segments - first draft, second draft and then maybe the final one. And thus we assign a deadline to presenting each draft. My problem is I cannot give logical deadlines. I am not able to assess how much time would a new task would require. Any suggestions on how to go about it?