The time I knew it was time to quit for the day was when I picked up my mini-iron by the wrong end - ouch!
The "timer" method works for me, too - I set it for about 30-45 minutes at a time.
My quilting triangle (cut-sew-press) is at three different heights, and spread out so that I can't just scoot around in my chair. I believe that just a little physical exercise can keep the brain sharper!