I remember one time I was working on a self-portrait for a class assignment, and I’d spent about five or six hours on it, and I was so close to being finished. I finally took a break, took a step back, and I asked a friend for her opinion. She took one look at it, and immediately pointed out that my proportions were all off. After we discussed it a little more, I realized that I had been drawing this big 3′ x 3′ drawing down on a table. I had been hunching over the table looking at the picture with a very fixed and specific perspective in that I’d drawn my face just completely squashed.
Now certainly, I’ve gotten way more in depth, and conceptual critiques than that. It really just illustrates the point that if you’ve been hyper focused on a piece for a really long time, it’s good to get a second set of eyes to take a look, and re-direct your attention to some things that maybe that focus might have blinded you to.