I work in the news media and "when it's done" works great for us, even when we're responding to breaking news. I make sure I break down my estimates and provide options on what can be cut or enhanced, and my project managers (I work on a couple small 1-20 day projects at a time) set their priorities.
It works great for all involved: I get to do quality work I'm happy with, and they always understand where their time goes. I admit it doesn't work with all PMs: some will try to mandate a due date. I'm really lucky to have a boss who'll help me push back, to explain that wanting things more doesn't make them go much faster. And I'm doubly lucky to have a boss who's right when he (very infrequently) has me work overtime to get something that's actually urgent done.
I find stress much less motivational than the pride in doing things well.