I usually work on a piece on and off during the course of a week or so, 30 minutes here and there, maybe an hour one night, a few minutes the next. I think it's great to take breaks, and then come back and crit yourself to death... I would actually say there's no better way to self critique, unless you've got multiple personalities... Also, if I find myself getting attached to how a piece looks, but notice something wrong, I just copy it into a new window, and I usually eventually end up throwing the old one away by means of compare and contrast.