What is the most effective maximum work duration per day?
This is a part-answer to your question and it doesn't come from occupational psychology, but from expertise research. 

Findings from Expertise Research

In their seminal paper on the role of deliberate practice for the acquisition of expert performance, Ericsson et al. (1993) report a number of constraints that play a role in the acquisition of expert performance. One of these constraints is the effort constraint. What is meant by this is that, in order to improve one's skill, one has to engage in deliberate practice, which by definition is an effortful activity that requires full concentration. It is clear that such an activity cannot be kept up for very large amounts of time. Ericsson et al. cite works by Welford (1968) and Woodford &amp; Schlosberg (1954) (which I haven't read) that compare the efficiency of practice durations ranging from 1-8 hr a day. These studies find that there is 


  essentially no benefit from durations exceeding 4 hr per day and reduced benefit from practice exceeding 2 hr. (Ericsson et al., 1993, p.370)


The authors then report that studies on the aquisition of typing skill by Baddely &amp; Longman (1978) and Dvorak et al. (1936), and studies on other perceptual motor skills (Henshaw &amp; Holman, 1930) which 


  indicate that the effective duration of deliberate practice is may be closer to 1 hr per day. (Ibid.) 


Finally, they cite a study by Pirelli &amp; J.R. Anderson (1985) that found 


  no increased learning from doubling the number of training trials per session. (Ibid.)


Thereafter, Ericsson et al. (1993) turn to more intellectually demanding domains. They report on data from financially independent writers, who 


  tend to write only 4 hr per day, leaving the rest of the day for rest and recuperation (Cowley, 1959; Plimpton, 1977). (Ibid., p.371) 


In their own study 1, which investigated violinists at music college, the authors found that the best students practiced on average 3.5 hr per day.

Maximum vs. Typical Performance

From these findings it seems that the optimal duration lies somewhere between 1 and 4 hours. But it's important to note that this only holds for conditions of maximal effort and full concentration. Furthermore these studies investigate the acquisition, not the execution of a skill, which presumably is more important in the workplace. I would still argue that they can still be relevant in a work context.

In io-psychology there is the distinction between maximum and typical performance. Everything I have said so far obviously refers to maximum performance. The intellectual work of a scientist that you mention in your question might fall into this category. I can also imagine that a software developer needs full concentration. Probably, though, periods that require maximum performance and those that "only" require typical performance alternate. The time limit for typical performance may be much longer.

So my answer is only a part answer in at least two ways: First it only provides information about the time limit for maximum performance. Second, the studies that I cite have not actually been done within an occupational setting.

References:

Baddeley, A. D., &amp; Longman, D. J. A. (1978). The influence of length and frequency of training session on the rate of learning to type. Ergonomics, 21(8), 627-635.
Cowley, M.(Ed.). (1953). Writers at work: The Paris review interviews, second series. New York: Penguin Books.
Dvorak, A., Merrick, N. L., Dealey, W. L., &amp; Ford, G. C. (1936). Typewriting behavior. New York: American Book Company.
Ericsson, K. A., Krampe, R. T., &amp; Tesch-Römer, C. (1993). The role of deliberate practice in the acquisition of expert performance. Psychological Review, 100(3), 363–406. doi:10.1037/0033-295X.100.3.363 PDF
Pirolli, P. L., &amp; Anderson, J. R. (1985). The role of practice in fact retrieval. Journal of experimental psychology: Learning, memory, and cognition, 11(1), 136.PDF
Plimpton, G. (Ed.). (1976). Writers at work: The Paris review interviews. New York: Viking Press.
Welford, A.T. (1968). Fundamentals of skill. London: Methuen.
Woodworth, R. S., &amp; Schlosberg, H. (1954). Experimental psychology. New York: Holt, Rinehart &amp; Winston.