Hey Emerson,
I tell a client however long I think it's going to realistically going to take me to finish a project.
I analyze my schedule, and give an honest answer. If it could take 24 hours... I'd give it to them in 24 hours.
But I've never been able to do that.
One reason I use this approach, as opposed to a cut and paste answer is, I know a client wants their copy asap. Therefore I try to give it to them asap. I've never missed a deadline, and God willing, never will. Also, I wouldn't want the integrety of the copy to falter based on a "false" deadline. I use "false" because most entrepreneurs don't have "real" deadlines. You know, like "the printing press starts at 3:00 Monday... and you better have your draft in"... kind of thing.
Here's an example:
As I type, I'm taking a break from actually "writing" copy. I do this every 44 minutes and 44 seconds. Then walk away for 15 minutes or so. It clears my mind, and allows me to work longer.
Eugene Schwartz said he could only work about 3 hours a day "copywriting" ... though somehow I think that was after he became "great". I would imagine in his early years he hustled like the rest of us.
So, right now, I'm working on 4 projects right now. So if I told a client 20 days... I really mean 20 days.
These 4 projects are comsuming alot of my time... and that's not even including the rewrite I'm doing for my own site, one clients entire marketing program (actually just did a launch and got a 51% conversion rate, and made him $226,000 in 72 hours for him

... and other businesses I'm launching on my own.
So Emerson, I tell a client the TRUTH. If the client can't work with the terms, I try to find them some help with other copywriters. If we're not a match, then I kindly tell them that, and hope and wish them success in their endeavor. But I'll never, and I do mean NEVER allow a client to dictate to me when I "can" deliver copy.
All because, usually this is based on a false assumption of a client. They think I have nothing else going on, and am taking their money and sitting on my duff watching TV or something.