I usually have 99 Hypers and start to use them on my party members halfway through Disc 1. I don't want to use Hyper before that since your characters' weapons aren't too accurate and you'll miss a lot, and you can't really afford to take too much damage before that. However, after my weapons are accurate enough and my party members can take it, they'll be on Fury throughout the game. It's very useful for getting limit breaks, especially in the forest battles around Mideel.