Halp! When to Use DPS Limit Breaks?

[Rongway]

Generally speaking, I have always found that your window of opportunity to use a LB closes during the second boss fight.

It doesn't.

if it happens to fill up again before the last boss (even if it's immediately before the last boss) to AoE LB the trash again.

In a typical last boss fight, the second bar will fill when the boss has so little HP left that a single bar would have killed him. But assume anyway that you would a second bar. A melee LB2 would not do as much damage as an AoE LB1 on 4-8 trash mobs. If there's no melee DD, an AoE LB1 on any trash group would do more damage than an AoE LB2 on the boss.

There is no such "closing" on the LB window. If there is a caster or ranger in the party, it is appropriate to LB any trash group of 4 or more. If there are only casters/rangers in the party, LBing anything that isn't a boss is better than LBing the boss.