There is no best practice. It all depends on your company requirements and how large you want the primary mailbox size to be.
Exchange 2016 Hybrid - Archive policy best practice
We have Exchange 2016 hybrid. For Online users we are going to enable online archiving. What are the recommendations/best practice to set the retention policy i.e. how often emails should move to archive database? 1 year, 2 year, 5 year? What is the best practice?.