If you exert yourself to the point of exhaustion when you shoot you'll likely develop poor form and bad habits. If you end a session of shooting when you can no longer draw your bow and hold steady on target, you've shot too much.

I shoot roughly a dozen arrows a day. Some days more, some days less. I feel I shoot enough to keep the muscles in shape and the form in decent working condition. I don't shoot in any leagues other than those I participate in here, so shooting any more than that isn't necessary.