as far as balance goes, MNK, WAR, WHM seem extremely underwhelming compared to the abilities you gave the other jobs.

If said LBs were to have their own bar separate from the party gauge than a good balance for time would be to make them so they can only be used once during a boss fight and twice (at the most) during a long dungeon. Given that, I'd say a 7min recast would be ideal (depending how fast the gauge fills).