It's the length of time in between the hour changing in the game. Ie quite a long time, of the order of 30 seconds. Even if I had to scan up (which I don't) it would take me about half a second at most between mob dying and attempting the assassinate. And that's from the uk.
I'm guessing you probably mean a round? Which is still about a second.
It would actually take about 2 pulses (about a 12th of a round if I recall correctly) plus transmission (data getting to and from your computer) and reaction time. No reaction time if a trigger is used.
But all in all very quick indeed. As an assassin I regular step in the moment a mob dies and land the assassinate before someone's last area spell they were spamming goes through.