A normal, human reaction time is that of just over a second. It varies, from person to person, and from training, but the time it takes to process what's going on and choose the best way to react generally stays around one second.
A tiny, split second in time, but a second nonetheless.
At any one time, a person is focusing on at least ten things; they have to be in order to process colors and wavelengths, depths, speeds and shapes. It takes next to no time at all to notice something unconsciously, but then you have to figure out what it is you just noticed.
Now, for someone who is trained to do so, all of this is reduced to a minimal amount as well as being conscious instead of subconscious; however, the brain is not capable of doing such things without nano seconds of time passing by as gears shift.
...Nothing really is... since... you know... it's time. Tree branch breaks, it still takes time to fall to the ground and crush you, y'know? Can't make it instant, just like you can't completely destroy anything (since the literal definition sortof means "to obliterate any and all traces of completely", and not "disentigrate to a fine dust nobody can see").
And of course, you have your prodigies, who have an inherent ability to do this at lightspeeds even without proper training. Then you have your prodigies who have been trained... who are really just kind of freakishly amazing in that way.
So! Assuming you've reduced your time by any of these factors, you're at about a .4 reaction time speed. If all, probably around .2...ish. All depends, really, it varies from person to person.
You've noticed, you've realized, you've said oh shit and you've begun to move (literally or figuratively) in reaction.
In the case of a fight, reaction time doesn't do shit if you're too slow to move your ass, but I digress...
In unique cases, where there's a lot to process (say, for instance, you wake to find your view point several feet shorter; there are a multitude of reasons why-- you could be on the ground, on your knees, everything could have risen for some reason, or you might have been turned into a furry black cat...), your reaction time is noticeably slower with good reason.
Also, if your brain is particularly sluggish for whatever reason, your reaction time is doubled if not tripled; sometimes, it can be increased to about ten seconds. Perhaps a minute if you're a particularly dim light bulb... but usually no longer than thirty seconds, and no shorter than four.
And so, it was about six seconds before Uchiha Itachi realized that he was, in fact, a furry black cat.
It was rare, at least in the public sense, that a single swear word would be heard from the elder Uchiha.
At this particular instance, in the safety of his own, kitty-mind, Hidan would have been proud.