Unless the game is defective (or you bring the game still wrapped to the same employee who sold you the game), you can't return new games at all. It's only used games you can return: 7 days for full refund, 30 days for exchange in case the disc ends up not working.
There might be a used copy in your local Gamestop; I know we have one in the store where I work. --- Oh! It's a signature!