You definitely can practice to much.Shooting a bow obviously is a physical activity,very often using muscles that are not used for any thing else.When you fatigue you are not as strong and your shooting form suffers.There are a number of experienced shooters that recommend that when your groups begin to open up it is time to either take a decent break or quit for the day entirely.
Personally I find that taking a day between practice sessions helps.For those that lift weights you don't exercise the same muscles every day, it doesn't allow your body time to recover.
A lot of guy's(and Gal's) think that more is better,and the frustration they experience by not shooting well pushes them to shoot more,trying to shoot themselves out of poor shooting.This is where the mental discipline comes in,you have to show that bow who the boss is,do yourself a favor learn this lesson early.
Quality is a lot more important than quantity! Take plenty of time between shots,and learn from every shot.And yeah,most important have fun,thats what it is all about!!