I figured I'd give you a "real" answer that didn't include losing body parts or eating bad mayo.
I've done a 3 day water-only fast before - I don't remember how much I lost, but it wasn't insignificant.
Depending on your current weight - losing 12-15 might be pretty easy (all things considered) or it might be pretty hard - if you aren't obese already.
Good luck with that.