At worst you should see 0.5s, don't forget the reaction time and all.
Never had hit by an AoE that I was sure I avoided since EU server are in EU. Even on ultimate I can move right before the end of the cast and if I "abuse" the server tick, a bit after.
Everything about battle is server side, when you know that and your ping you can do everything without trouble.
btw there is the animation for some skills (player or monster) that aren't sync with the mark on ground. You can be hit by an AoE even if you were out before the mark was gone, simply because it was a skill more based on the animation of the boss. See the cast or mark as a indication that tell you the next move or the size of the AoE. If you didn't have those indication you'll probably avoid them more easily after learning how they works.
FF isn't a game about reaction, but learning what's happen next.