What determines the stagger duration?

When I fill the chain bar to stagger enemies, will the bar, once it's full, always deplete at the same speed or can I somehow keep the stagger bonus up for a longer time?
I have noticed that the stagger duration is always much shorter right after a preemptive strike. My subjective feeling is that the duration has not always been the same when I fill up the bar during the fight either, but I might just be imagining that and wouldn't know why.
Attacking with a Commander slows down chain depletion before the stagger, does it also increase the stagger time? How is the length of the stagger bonus determined and is there anything I can to to increase it?
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Lost Ark Mechanics Guide: Counter, Stagger, Weak Point
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