Generally you don't even get 1 bar before the first boss dies, and if you use your bar on the 2nd you won't fill it before reaching the final meaning you only get to LB1 it.

This means the optimal time to LB is between the 1st and 2nd boss, generally ranged DPS on a mass-pull. (or for SMA melee LB on the wall)

The only exception is on a well-orgainzed party (speedrun strats), you can force-charge LB if the healer allows the tank to drop below 10% on the first trash pull after each boss before healing. Each time they do this, it adds a gradual charge effect to the bar that doesn't stop until it fills. I know people complain about healing during Living Dead, but it's a perfect opportunity to grab this effect if the healer is good about it.

And yes, immediately once LB2 charges on final (or at least the first break in rotation you get), or DOESN'T charge because you're that fast and LB1 can finish, pop it.