Actually it should be given after 4½ - 6 hours !
Just last Friday I had my Presentation on this ! 
my doctor taught me this way
No idea about new variants , but ideally it’s 6 hours and the answer that is gonna get us 1mark is 6hours !
It can be given within that time and not after. Plus that’s approved only in certain countries. But all over the world it’s approved until 3 hours. Let’s just clarify it with some faculty for our exams!