I weigh myself first thing every Saturday morning and I don't stray from that. I used to weigh in the middle of the week, but it can be incredibly demotivating for me since my weight can really fluctuate from day to day. I would work super hard for a couple days, then weigh myself because I would get excited and find that I hadn't lost anything, or only lost an ounce or something and it would frustrate me. Basically, I figured out that doing this and getting disappointed was a trigger for my binge-eating, so I promptly stopped. I even resisted weighing in after a two week vacation (where I gained TEN pounds) when we got home on a Wednesday.
I'm pretty strict with myself - the only time I'll stray is on Saturday or Sunday after I've had a long work-out and I've sweated a lot, I get on the scale to see how much water weight I've lost, hah! But that is really infrequent and I only do it to see how much sweat weighs.