Actually, there is no defined limit to say sleep is too much. Sleep is an essential process for the brain to rearrange synapses in between neurons and form memories. It is also important for registration of what we have learned in a proper order.
In addition, the body also gets a time to repair the minor injuries which occur in day to day life. The growth and many hormonal activities occur during sleep.
Usually it says 8 hours a sleep is enough but time can vary from person to person. Unfortunately the upper limit is not defined.