RE: how much is too much?
It's one of those things where it boils down to the individual. One of the absolute best shots I know (Rod Jenkins) said he shoots 4 hours a day or more. From the callouses on his fingers, I believe him!
For you, here's my advice (take it for what it's worth). Don't shoot until you are worn out--you will get sloppy and develop bad habits. If you start aching or hurting, you are overdoing it. If you can't focus on the spot, you are overdoing it. If your groups starts spreading, you are overdoing it.
For some folks this might mean they don't shoot more than 15 minutes at a time a few times a week. Others may be able to shoot several hours a day every day. You have to use your own judgement and be honest with yourself. Set goals, but be realistic and listen to your body. Don't make yourself shoot a certain number or arrows or a certain amount of time if your body starts telling you it's time to quit.
Chad