This is something people get confused with because of the travel time of Gpy.
Let’s say you are casting Gpy at a warrior. Even if you’re 55 yards away with glamecannon, if the warrior spell reflects at ANY TIME after the cast goes off, it will hit him. He needs to throw up spell reflect right before the cast finishes. Although the animation of Gpy is slow af and the spell reflect buff would theoretically fall off by the time the meatball hits you, the Gpy would still be reflected. This is the same with lock reflect, prismatic cloak and any defensive.
So people that are trying to figure out the travel time and the optimal time to spell reflect or pop a defensive… It’s just like any other spell