When I am aware things aren't going well. Sometimes that takes awhile... but I've found that things never just start going better for me without a break. It sucks when you are on a real deadline but with the way I try to structure my quilting I have very few of those.
Yesterday or the day before, I sat down to put on some simple WoF borders onto a panel. After the fourth time I had issues with the threading with the first 18" of the first seam and just as I was starting to get a tad excited, I told myself the same thing I tell other quilters. When you can't figure out why something is happening and you are sure you are threaded correctly, change the needle. So I changed the needle which had worked perfectly fine the day before, and suddenly Bernie (the sewing machine) was just happily sewing along.
When in doubt, rethread first. Change your needle second. And if that doesn't help, take a bubble bath, get a drink to re-hydrate or caffeinate yourself, walk around the block, or just pour yourself a stiff shot if that works for you. And there is always chocolate!
Studies have shown that about 20 minutes of break is key for a number of reasons. Some times though keep in mind that is the minimum and things will indeed be better tomorrow. That what I like about a bath, by the time I take off my clothes, let the bath fill up, play in the bubbles, and then let the bath water out again, and then get dressed again it's a good 20-30 minutes. I was first given this hint for dieting btw, it also allows you to get in touch with your physical body because many of us have a self/body disconnect.