Kepler's Third Law implies that the period for a planet to orbit the Sun increases rapidly with the radius of its orbit. Thus, we find that Mercury, the innermost planet, takes only 88 days to orbit the Sun but the outermost planet (Pluto) requires 248 years to do the same.

Here is a java applet allowing you to investigate Kepler's Laws, and Here is an animation illustrating the actual relative periods of the inner planets.