Why don't authors include in their publications the duration of the research? I've noticed that most papers do not say explicitly (or at all) how much time did the research take. This excludes certain types of studies, such as epidemiological studies that usually say that the study's cohort was followed for some-and-some months etc., but what I mean is that in most papers there is no actual indication of how much time the study\set of experiments took.
I believe this type of information could be useful for readers, especially for readers that would like to perform similar experiments for their own research.

So why is this information not required from authors?

(I realize that it's sometimes hard to pinpoint the date when a person begins and finishes a research project, but it could still be useful to give an approximation in months. This way a reader could get a better estimation if such a project is reasonable for their own research, at least from time investment considerations.)
In a paper I wrote 35 years ago, I remarked that there ought to be a strong connection between a certain pair of concepts. After doing nothing with that idea for a long time, I recently worked out the connection, in a few days. I'm about to start writing this up, but first I had to check the relevant literature in order to cite it properly. One item was available only by inter-library loan, and the time spent waiting for it was roughly similar to the time spent actually proving the theorem. If someone asked me how much time it took to do this research, I could truthfully answer "a few days" and I could truthfully answer "35 years". 

Furthermore, neither of those answers would be useful for my readers, because confirming my result will not involve repeating experiments but rather just reading and checking the proof in my paper. How long will that take? Maybe an hour or so for an expert in the relevant area, but much longer for someone unfamiliar with the area who needs to read a lot of prerequisite material first.

So I'd say that, at least in mathematics, information about the time required for research is likely to be meaningless and useless.