When you ask "
how often the client is retrieving the Cluster Map?" . The obvious answer to that is there is nothing 'often' in it. Whenever there is a change in the map, the monitor will inform the client.
I think you need to read about the CRUSH algorithm in Ceph. Because that will explain you the map changes and data movement.
While going through CRUSH, forget there is a monitor node. Just suppose there is a client machine and this client machine READ/WRITE to a cluster ( number of OSDs). Because theoretically a Ceph client can also be a monitor (not at all recommended for practical purposes). Once you have understood CRUSH, I am quite sure that will answer many of your questions.
And feel free to ask about CRUSH. I would be glad to answer.
BR