The buff time for Prefulgence is 30s, and that doesn't start ticking until you spend your last Manafic stack. Which just got buffed to 30s duration. That means you have nearly a full minute until the buff falls off and you lose the cast. Considering it's an oGCD, you can easily work it in between any number of casts between when the Manafic timer starts ticking and the Prefulgence buff falls off. Obviously you aren't normally going to nor should you want to drag out Manafic's buff timer to its almost full length, but even if you rush it, that's still a good 10~ seconds spent burning all 6 stacks before the Prefulgence buff pops. That's more than enough time to hold it for your next Embolden window for the majority of fights; and any fight that would go long enough for that to start to become a concern is almost certainly a fight that you'd already have a planned/expected killtime for, and therefore be able to know if/when to hold your Manafic.

Although the cooldown on Manafic is annoying at times it gives the job just the right amount of breathing room in its kit to be able to adjust on the fly, you can rush or hold it depending on whether or not you'll know you need mobility, will have to disengage from melee range, or if you're going to cap your mana. It adds a good bit of nuance when a lot of jobs have had their rotations tightened up or made even stricter with timing.