So is there a source to this magic?

If it's internal, how quickly does it replenish?
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[*]Like, if I throw a fireball, am I waiting a few seconds, a few minutes, an hour, a day, a month...?

[*]Does it accrue faster if I eat a piece of hard candy? Eat a candy bar? Chug down a monster energy drink?
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If it's external, is it like tapping into a ley line?
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[*]Filing up a reservoir?

[*]Live sacrifice? (So, uh, nice altar. Those knives look so very sharp (and bloody)... Oh, is that the time? I... ooh. Aheh, this door is locked, can you... why are you wearing that robe?)
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So like vampires have a lot of mana after feeding?