It would seem that once you cast you use up mana, BUT from my observations, if the mob dies the mana is not charged ... it appears that you don't get charged the mana until after the spell hits, and if the mob is dead it dosnt hit, but i have seen spells charge mana upon cast so, but if you cast a spell and get interupted you dont get charged ... ...

Of course maybe I am regening the mana back just as fast, Ill try and nail this down with SS.