I don't think there would be any, for the simple reason that you can't study it objectively:
1. It varies from project to project
I had to work on projects so boring that working one hour per day was already too long. With badly written requirements, badly written existent codebase and no quality whatsoever required from me, I could try my best to concentrate, and find myself browsing Programmers.SE or doing other unrelated stuff for the last three hours.
I also had to work on projects so exciting that I spend 60 hours in a week on them (without being forced to), writing not so bad source code.
In those two cases, I was the same person. In the first case, my productivity was terrible and I was able to concentrate 0 hours per week. In the second one, my productivity was high and I was able to concentrate more than 40 hours per week.
2. It varies from company to company
As an experience, you can put two developers in different working conditions on the same project. One developer will work in her own office with a dual screen fast PC, comfortable chair, etc. The second one will have a desk in the middle of a call center, with an old PC, a 56k internet connection and a 50 MB limitation for all the personal files (and no right to use USB sticks).
Two weeks later, invert the working conditions. See the difference?
3. It varies from day to day
Imagine that on Thursday, the developer knows that he will be able to quickly solve the remaining issues, that the work to do is interesting, and everything is exciting and promising. She also learnt that she was promoted, and her husband contacted her to announce some good news related to her personal life.
On Friday, the same developer finds her old dog dead and her car won't start. She's late at work and spills her coffee on her desk, etc. How would this affect her daily performance, compared to the day before?