In learning theory and skill acquisition, research supports that several shorter sessions equate to better skill/content retention that one long block of time. So perhaps its not the amount of time, but how you divide that time.
Also, its about the quality of that time. Many people spend many hours at the dojo and on the mat, but not really training (more socializing). So you may also have to qualify the intent and intensity of the quantified clock time and number of days.
As in running, more is not necessarily better. Too much, too fast, too soon often leads to overload, lack of progress, and early burn-out.
Human engineering for skill acquisition is an interesting topic. Please keep us posted on your findings.
There was some other recent research I saw in an article, though sadly I can't find the link, but it said more or less the following: learning sessions must be sufficiently spaced out that you are forced to expend effort recalling what you did in the last session, because this recall effort strengthens those particular memories.
So spaced out sessions rather than constant practice allowed for that recall effect to come into play more often. But at the same time, it was a balance, because if you spaced those sessions out too long, of course it was also detrimental to learning.
It was also mentioning about how it was best to train related skills in a circuit, rather than a long block on each component, then the next component for a long block, but rather bring up all the skills a little bit at once, which would seem to allow this recall effect to better come into play, while also allowing for better integration of all the practiced components.
The research was studying violinists, and how the elite vs. merely also-rans spent their time, and surprising the elite level violinists spent somewhat less time practicing, and their training was more compartmentalized in the day. When they were not practicing, they simply went about their lives and didn't worry about the violin. Whereas the also-rans seemed to if anything be a bit more obsessive about their practice and get much less out of it for the same amount of time spent because it was not as well compartmentalized.
Train smarter, not harder, I guess. So long as you train enough, but enough is certainly probably not as much as sadomasochists would like us to believe.