Why don't authors include in their publications the duration of the research?
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.