I always try to be aware of my marginal impact/work ratio. I know that at a certain point in time, an extra unit of work will produce an extra negative impact, or diminishing returns. At that moment, I try to switch my work so that my marginal impact is positive.
That point in time usually is some sort of physical fatigue, boredom, mental distraction, or lack of inspiration. Sometimes when that point comes, it is almost better to sleep or take a nap to recharge.
I don't usually try to push my upper bound because I know my productivity will suffer. I know that I could push my upper bound if need be, but in most cases that is not the most prudent action.