I wait for the bar to fill, then I wait until the best opportunity. Don't want to use the melee LB if there's a group of adds or something that need to be downed (like in Garuda HM) by the caster LB, and I don't want to waste the LB if it's better saved for a healer LB when the party is struggling. I also don't want to be stuck waiting for the LB to go off then get hit by the next landslide in Titan HM (learned that one the hard way). Those are the reasons why I wait. Usually I'm able to use it as soon as the bar is filled.