I haven't found the perfect balance. It's tricky and I think it really depends on your kid. Some kids thrive on being perpetually busy and others it just saps them. The problem is what can help one kid doesn't help another, there really isn't a one size fits all.
Most important is listening to your child, communicating with them, and trying out different things. You won't get everything right all of the time. I once totally by accident over-scheduled my daughters after school activities, it was a crazy 2 months we got through it and I better learned a lot from the experience.
For my son I have overstepped and pushed too hard in 6th grade, we then backed WAY off. Now that he is in H.S. I am back to pushing him more than is natural for me, but I feel it's what he NEEDS at this time. I am having a hard time getting my son to understand that many of the other kids aren't "better" than him they just work a heck of a lot harder.
