Since the topic is LBs, I thought I'd also mention something that's generally a none-the-wiser mistake, and that's waiting to use an LB till the very end, when the maximum damage of the LB is wasted on a smidgen of health, and when there's no more DPS to mooch from a mini-boss or boss to get as much bar back as possible.

Others here have pointed to particular situations where LBs are crucial to use, but outside of those moments the general rule of thumb should be sooner rather than later. Especially on the first and second mini-bosses of a dungeon.

The bottom line is that the sooner someone uses an LB, the sooner the bar fills back up and it can be used again. With this as your starting point, you should then look at crucial moments, or the most useful moments. A party with mage only DPS and therein AOE only LBs would be best to use the LB sooner, but also when adds are spawned once the tank has collected them, or when a boss has a secondary part that needs dealing with, like the scorpion's tail in Toto-rak (to give a low-level example).

Over and over again you'll see people waiting until a boss is at next to no health before using the LB. This is a total and utter waste of DPS, especially when used by melee DPS.