When they flash me it feels like some good 4-5 sec, but when I do i get insta-killed as if they weren't flashed, or they are flashed for like 2 sec I don't know what am I doing wrong

As the title implies, I am a new player coming into this game and so far I like the game so much. Although it's been almost 1 month since I created an account I still have some questions, some being gameplay related, others being more miscs...