time to access Office 365 Message encryption messages

Mail is not stored in the service, but in your (and the recipient's) mailbox. O365 servers are only used to decrypt/display the message. As long as the message is not deleted and you are still using the service, it can be accessed. If you mean message expiration, I dont think this is officially supported, though we do have a parameter for PowerShell that hints it's possible.

The service does store the encryption keys however, and as OME is using Azure RMS on the backend, decommissioning of the service will be similar to what's detailed here: https://docs.microsoft.com/en-us/information-protection/deploy-use/decommission-deactivate

Not sure what you mean about control? Did you check the FAQ here: https://technet.microsoft.com/en-us/library/dn569285.aspx