Shisui's Mangekyo Sharingan Recharge Period According to Narutopedia for Shisui's MS ability Kotoamatsukami...


  Itachi noted that the dōjutsu required at least a decade before it could be used again.


Does this limitation apply to Shisui, or does it just apply to those that took his eyes (Danzo and Itachi)? Because if this applied to him, then he's not as OP as everyone thinks he is. Plus how would he know it has a recharge period of 10 years?
To answer your two questions


It was never mentioned if Shisui had to wait for a decade to use it, so we don't know if this rule applies to him. If he did have to wait for 10 years, he's very underpowered. I don't think it applied to him because everyone knew about his ability and if that rule applied to him, he'd have used the ability for a maximum of 2 or 3 times (not possible, 2 is max because 2 eyes), many people wouldn't know about it.
Itachi said that Kotoamatsukami takes 10 years to recharge for those who don't have Hashirama's cells.
There are 2 ways he could know the recharge period.


Shisui told him. That would mean, this drawback would apply to him too. But its very unlikely that he had the Mangekyo for over 10 years. So he himself wouldn't know about the recharge period.
He used Kotoamatsukami just after Shisui died and he knew the period was 10 years because it activated after 10 years in the war. This is dumb. I'm dumb. Itachi is smart. He wouldn't gamble.
The third way, I know I said two but bear with me. THIS IS A PLOT HOLE. Every anime has one. Itachi just couldn't know the recharge time.



THOUGHTS

How does Hashirama's cells affect the eyes? If we give the eye which has been used for Kotoamatsukami to say Danzo/Yamato (someone who has his cells), would the eyes recharge instantly?