At some point, working more hours makes you less productive. As an obvious example, you can’t work literally every hour of every day, since without sleep or food you can’t function. Now perhaps the curve for you really does peak at 100 hours/week, or 80 hours/week, but frankly I doubt it.
Acknowledging that there are thresholds for productivity, it also dramatically depends on the type of work and the objectives. Diminishing returns are bad for some tasks, acceptable for others, but both are subject to the types of deadlines. Implementing a half-good feature just in time for a big marketing event might be worthwhile or it might not, but cold calling potential customers to keep from failing generally is, and is somewhat less subject to declines from productivity loss.
At some point, not only do returns diminish, but they actually become negative because your productivity during the first forty hours of the week goes down when you don’t get enough sleep, relaxation, etc. Perhaps that point is at 100 hours/week, or more, but I’d wager that it’s at the 80 hours/week mark where working more decreases your productivity.
And yes, it does depend on the activity, but even cold calling customers all day can become too much, or an extra ten hours of calling could make your sixty hours of programming less productive.
