How long will it take to migrate a mailbox?
It is not possible to provide a true estimate of how long a migration will take due to numerous unknown factors such as the load on the server, available bandwidth, network congestion, etc.
To increase the speed of the migration you can try excluding your Deleted and/or Sent items during the first run. Once the migration has completed for the other folders, run the migration again and exclude everything that was included in the first run. You exclude specific folders by enter them one per line with the same name and path as they appear in the mailbox (for example: Inbox or Inbox/reports). The subfolders of any folder you enter to exclude are also excluded.
If the mailbox has a large number of older emails, you can also try first migrating the email from the past 60 days and then migrate the remainder.