Before picking up Fable II, you need to try and forget everything you've learned about magic from other role-playing games. Fable II does not use mana. There is no cost for casting any spell. You could (and are encouraged to) tap the B button repeatedly to fire off a quick succession of attacks at enemies. Doing this, however, only casts level 1 spells. So while it can be effective, you'll also be doing minimal damage. To get the most from the magic system, you'll want to use an even mix of quick-firing spells and charging up for larger attacks.