According to this article, around two ten-thousandths of a quadrillionth of an ounce...
(i.e. not a lot!)
Go take a read - it's quite amusing and desperately geeky :)
J. K. Rowling: Harry Potter and the Deathly Hallows (Book 7)
A satisfying conclusion - read while I was convalescing from the broken nose, a much-appreciated side benefit to being injured! (****)