Maybe instead of measuring tasks/projects in man-hours, how about mind-calories? For example, 100 mind-calories is equivalent to 1 hour of work that is of the mental intensity that 60 hours of it in a week would make you too tired to do anything outside of it except casual social and leisure pursuits. i.e. The average American white collar worker uses 4,000 mind-calories toward a job that he or she is okay with, and has 2,000 mind-calories left for errands, commuting, and dealing with other stresses. If he or she tries to use more mind-calories, then he starts to get worn out, agitated, and possibly neurotic.
Right now, I've for example, scheduled myself to do 30 hours of work on iPad development a week. This seems totally manageable on the surface, but it really depends on what I'm doing during those 30 hours. If it's something like programming, which is a 150mc/hr activity, then those 30 hours are actually a 45-hour work-week. That's one end of the spectrum. At the other end, let's say, I'm doing mock-ups in Photoshop, which is something really fun, its more like 50mc/hr. And then if I'm doing QA with my friends, I actually gain energy back, and so that's like -20mc/hr.
I think I prefer these kind of units as opposed to man-hours, at least when making plans for my weekly schedule. Sometimes I take on a task that seems harmless enough, and only requires 1 hour of work, but the stress and agitation involved may actually take out 500 mind-calories.
This idea also explains to me why you see perfectly smart college graduates becoming "starving artists" waiting tables while working on their real craft after hours. It's because waiting tables is a mindless activity, maybe 50mc/hr for most people. I always wondered, "why don't these bright kids get some nice-paying marketing job during the day, then work on their hobby in the evenings and weekends?" But this is why. There's no juice left to be creative.
I really love this concept of mind calories as far as being a metaphor for being exhausted. But it also made me think that if you burned mind calories, you wouldn't become 'lean'.
I think that one of the really brilliant things Scrum does (not to Proselytize too much about project management methodologies) is abstract man-hours into 'story points' which can go along the Fibonacci sequence, or get abstracted into shirt sizes (sm, med, lg, xl, xxl) because the bottom line is that people are terrible at estimating time. But they're great at estimating bigness or complexity! The ironic thing is that it's actually time estimation (using man-hours) which is abstract because it's like trying to overlay a straight line over something that's clearly a squiggle.
True that, I really liked the Scrum Fibonacci weighting system when I was under it. Because it's a better estimate about what a programmer can do. Even if he can do the task in 10 hours, if it requires too many mind-calories, he will take longer on other tasks. A bunch of light tasks on the other hand, can always be compressed into less and less hours.