How many hours it takes?

Mihile is a godsend for adv mage classes as it pretty much means free 100% stance versus basically none at all (because who praises infinity for the stance effect) due to free 50% buff duration. For every other class, it's garbage due due its CD relying of massive amounts of buff duration in order to make it useful for the classes that would like it. The only way to get it is through Inner Ability or Monster Life (which many see as a waste of time; I like it )

I disagree, mikhail link is very good even if you have 80% stance like i do, but only in the cases where super KB exists. Also you don't need "massive" buff duration to make it useful. A single mech card cuts the cooldown a significant amount and there is literally no reason not to have a mech card in your deck regardless of class.

Merc/Mech - makes buff skills with CD more useful *cough*AB*cough*Mihile*cough*
Battle Mage - If/When the damage reduction applies to %HP based attacks, it will be a must. For now, it's garbage.

Merc is even more useless than cannoneer card. It isn't worth using a card slot for the purpose of shaving seconds off of 1 or 2 at most important skills.