It is pretty common that we'd want to automatically change storage classes of an object, so that it stays in a hot bucket for say a month, then gets relegated to a colder bucket, and finally is deleted altogether. This is what object life cycle management policies are meant for.

They allow you to specify how long the objects should exist under the same settings in your bucket before a specific action is triggered. For example, when we realize that there is archived data that has not been modified for over six months and is not likely to be accessed or modified any time soon, you can convert their class to nearline storage to save on cost.

You should note, however, that object life cycle actions are classified as Class A operations, which means that they can be expensive if you don't use them right. Do check out the fine print...