sry, but i can't tell u the time-limits for each zone target, b4 it says 'target depleated'.
but i guess 55target has a limit of 5min, maybe. have to try this later...
edit: after testing a bit at ZB55 syncopia (novice AI), i found out that after 7min20sec the zone-target starts to depleate.
i never used the boost in my tests, instead i absorbed all energy. 1.0 zone in ZB = 20sec! in normal zone mode 1zone = 10sec!
after reaching 22.0 (without using boost once) the target began to drop.
lets do the math: 22.0*20sec=440sec
440sec/60=7.33min=7min20sec