There is no real cooldown for every boss but the mechanic you need to make on each boss can be "cooldown" of him. And there is one important thing that you have to leave room everytime you kill the boss and just go back to the room again.
MARZIEL - All you need to do it use vial of blood on carpet and there is no cooldown (remember it has to be one female character)
BORETH - you need to use 3 times fire bug on trees and sometimes every tree will burning so the cooldown here is to just wait until they stop burning to use 3 times fire bug
LERSATIO - on this boss you need to destroy wall mirrors in the room and everytime you kill the boss you need to make the same so you need to wait until they repair, it takes around 1min-1min30s
ARTHEI - and there is no cooldown on this one
And of course we can count as "cooldown" the way you need to make from first floor in vengoth castle to boss, it may take some time (for example on arthei)