Not currently available at my PC but this is the setup you can use to test it:

1. Be in party with someone having a 2min LB (PLD SGE SAM or another BRD)
2. Charge up Final Fantasia, have the other person not be in combat with a striking dummy.
3. Have person now enter combat then use BRD LB
4. Keep track of the time until they have LB, subtract when they have it from 120s, that's the value of full duration LB boost.