I work better when things are neat and organized. Just today I finished a stack of Christmas presents and I'm pleased as punch I can walk through the doorway to my bedroom again. (had to use the other door for months now - hooray!)
I can tell when it's time to clean up when I can't focus on any one particular project. Sometimes I'll just sort my scrap bin (by color) to "feel" better and move on. Right now I have fabric for two purses on the floor that have the interfacing ironed to them - they need cut out and then sewn, and I'm working on a full sized quilt (with pieces for each block cut out and in seperate baggies). When I get those two done, I'm going to clean out my UFO stash (I lost count there).
I feel so much better when things are organized. When I get my UFO stack(s) done, I'm going to make something out of my scrap bin and get it all cleaned out. After that, I'm going to start on my "projects" (quilts/bags that the materials are purchased, but I haven't started) -- hopefully by the time it's time to start christmas presents, I'll have a super clean and organized studio!!!