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I think it would be easier to ask how he know it could only be used every 10 years since he clearly had it for less than 10 years...
I think this should be in Naruto Toshokan
I think that it's not only his technique but has been in someone before in the uchiha clan, a predecessor.
Look at Itachi's amaterasu = people knew that it was black flames that burnt everlasting, even though not many should have seen or maybe known about it.
Then you have the sword that Itachi's susanoo used. It was a sword that orochimaru searched for his whole life. But at the end Itachi had it.
So I think that this abilities have been before our time, and the info about them is/was always there for those who wanted to know.
and i am sure that the tablet under the shrine has all the information about uchiha and there eye powers so i am sure they are all in there
This suggests that even Shisui was affected by that time limit
Itachi's a genius, he could have easily made a hypothesis using whatever data he had. Wouldn't be the first time a genius in this series figured something out with next to no evidence.
Uchiha Shisui didn't have that time limit snce it was his Mangekyou in his body, but Itachi isn't Shisui and Shisui's MS that Itachi had wasn't in its original body. To use it more often one would need Shisui'S or Hashirama's super cells, both of which Danzou. But since Danzou wasn't full Shisui nor Hashirama, his use of it was still limited in time, but not as long as for Itachi, as he used it in the summit and, from what Tobi said, was hesitating on whether or not to use Kotoamatsukami on Sasuke os use the Sharingan for Izanami. But Shisui shouldn't have had time trouble using that jutsu.
In addition, Kotoamatsukami being Shisui's MS would have meant it wouldn't have costed him that much chakra to use. Look at Itachi, who while confirmed to have low chakra levels, is still capable of using his MS multiple times in a single battle. Or Sasuke, who uses his MS continuously without worry. So Shisui would not have been limited in his usage simply due to chakra. Aoi, Kirabi, and Kabuto all made it clear that Shisui was infamous for Kotoamatsukami.