From Motoko Rich's article in today's New York Times, New HarperCollins Unit to Try to Cut Writer Advances:

Typically, authors earn royalties of 15 percent of profits after they have paid off their advances.

There are a couple of errors in this sentence, astonishing from someone who covers publishing for The New York Times.

1. First of all, royalties paid are not a percentage of profits, but of the list price of the book.
2. Second, the word "typically" is also incorrect: 15% royalties are typical only of million dollar writers in paperback where it is otherwise customary to cap royalties at about 10%, period the end.  And in hardcover 15% royalties occur, when they occur at all, only on a really big book only after you've sold really a lot of copies. This is unusual, not typical, much more often stated in a contract as a possibility than occurring in fact and on royalty statements.
3. Earnout of the advance is not sufficient to trigger a immediate 15% royalty on the vast majority of books published.