Sometimes even when I define what done means before I start a project, I embarrass myself with my underestimates of complexity. Software is just plain fickle. Proof of concept on most any web app? Weekend. Something that satisfies anyone but me? At least two weekends :)
The best time to define done is when the time period has expired. Just take what you've done, list that as your acceptance criteria, and viola! You're batting 100%.
