Dear Randall,

For what it's worth, 45 minutes seems about the length of an average set - a span of time after which both performer and audience often appreciate a short break of say, 10 or 15 minutes. I find that if I play/practice for 45 minutes to an hour, then take a break for a bit to drain off, top up, yawn, stretch and wander about (outdoors, weather permitting!), I return with renewed concentration and vigor.

I find the same sort of pacing helps when I'm writing or doing other work at the computer, too.