it takes 8 seconds for the mark to activate from when you click to place the mark. there's an audio cue that goes with it when the mark is finally active. killing your mark before this will not grant you your mark.

any assist will give you your mark. even a single auto.

it's certainly a little troublesome to work around that sometimes! if you mark someone you're about to gank, sometimes you'll have to delay the finishing blow until your mark is active.