One of the biggest mistakes people make when trying to build muscle is doing too much. They figure that if three or four days in the gym is good, then five or six is even better. But muscle development is a process of tearing down and rebuilding fiber. If you work out all the time, your body doesn't have a chance to restore itself. Instead of going from skinny to buff, you'll find yourself at a very steady level of underwhelming fitness [source: Haycock].
Give yourself 24 to 48 hours between workouts to allow your muscles to restore themselves. The time off will allow those tiny muscle fibers to rebuild, making you bigger. Without the rest periods, progress will be slow.