Quote - I think it's also a little subjective as to what a person considers a long time.  To me it's nothing to leave a render going all night or even into the next day when I'm at work.  But I've had friends who thought anything over 20 minutes to be too much.

Depends on the trial and error involved in a given project.  There's nothing worse than waking up and finding something unexpectedly wrong with your all-night render.

If you are doing animation, even 20 minutes is a long time.