Board Thread:Theories and Speculation/@comment-36512555-20200320190539/@comment-32685487-20200321200501

It also depends on who is casting the Amaterasu, Itachi has shown he can make them burn faster with accelerating them, near instantaneous while in moments where he isn't actively controlling the flames they are burning at a normal rate such as when they were lingering in the forest around the Uchiha compound, I don't recall Sasuke ever making something burn away near instantaneously.

And at the speed those bones fly it would HAVE to be near instantaneous to stop them from making contact with the target.