Maybe I cannot speak for everyone, but two or three hours is really about all I can take before I need a break of some kind (to stretch, to eat, whatever), and

Maybe I cannot speak for everyone, but two or three hours is really about all I can take before I need a break of some kind (to stretch, to eat, whatever), and at that point you end up switching contexts anyway, so you may as well jump into something completely different when you return. It is no more costly to "load up" something new than to "load up" what you were working on before once the switch has happened. Now in the real world, where people expect you to do just one thing, I often do multiple two, possibly three, hour stints, with context switches in between each. However, I feel a lot better on the days when I'm able to do different tasks on each switch rather than coming back to the same task each time. You, naturally, still have to give that task your undivided focus while you are working on it. Trying to manage everything at the exact same time will definitely lead to what you suggest. Of course there are no hard rules. If, for example, you're having a good programming day, maybe you'll put in four straight hours before you feel the need to stop. It is not like you have to put your metaphorical pencil down as soon as the clock strikes the two hour mark.