If you don't know how far away you have to be from the bomb, look at your mini-map while running/making noise... Once the bomb symbol is outside your circle you are far enough away from the bomb explosion.

I noticed this when i started Valorant, that pro players (obviously) have better movement/gamesense/aim then me. Now i am much better then before and while my gamesense are (obviously) not as good as pro players i am confident in my ab...