How long is the champion's kill debuff on a certain dying enemy champion?

Sorry for the misunderstanding title.

I mean as in the time when the enemy's last hit goes off before the target can die as an "Execute" instead of being slain. Cz as I checked out the video here, at 5:37, Varus died after being unsuccessful in killing Soraka. Then after for like 13 seconds, she got shot down under a turret and got slain by the Varus, not executed.

#13 seconds

https://www.youtube.com/watch?v=mkBED-HugRM

So can anyone tell me what's the duration for the killer's last hit debuff to go off before the target can be executed once again?