From what I have read MP regenerates at a speed of 6% every 3 seconds when out of combat, and 2% every 3 seconds while in combat. I think.

And what do you mean by differ the regeneration? You can use potions to restore a % depending on the strength of the potion, or you could increase your Piety which would increase your max MP thus increasing how much MP you regen every 3 seconds. Other than that Scholars have Aetherflow and Energy Drain, and WHM have Shroud of Saints. I'm not sure what else to tell you about it.

EDIT: Oh, and Bards can help regen MP faster with a Ballad.