I think it lasts all their lives, but Joey's learned not to chew on my stuff. I'm not sure they are completely through teething at 9 months, are all the adult teeth in? Dogs do continue to have a need to chew even after they are through teething. Besides bully sticks we also got one of those huge chewing bones for Joey.
http://www.yorkietalk.com/forums/pic...pictureid=7724. Use apple bitter or clear deodorant for things not meant to be chewed on, and limit access to the house. Your kids are going to have to learn to keep their things picked up, and no matter what age, they are going to want to chew the wrappers from McDonalds, they can smell the food. I've taught Joey to keep off the coffee table, but if your kids are putting things on the floor, it might be easier to teach them not to do this, then it is to teach your dog, not to go for it. It's a toss up.
Remember a dog cannot understand the difference between their toys and the children's toys.