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.)