I have to agree that sports is a positive activity and outlet for children. It gives them a healthy outlet for all their energy and teaches teamwork, perseverance, resiliency. My own children play hockey, baseball, take swim lessons, soccer as well as play sports for the school. I wouldn't want it any other way.
I thnk it's ok that kids should at least play on weekends and the parents can join them in playing too because they don't have work that day. You will know when playing is too much when they start sleeping way past their bedtime or their grades start to drop slowly.
I'm only at the toddler stage right now so nothing my daughter does is time consuming. She has soccer once a week for an hour. But it's very good for kids! No matter what the age. It keeps them out of trouble, helps with their fitness, keeps them healthy, and you know where they're at so you don't have to worry. You can know they're up to something beneficial and not something bad. It's great for them and I don't think there's really a limit. If it's becoming obsessive because of deeper issues then there's a problem. But that's a whole other story. If it's just because they enjoy the sport, then let them.