Following the steps at this link:

https://docs.microsoft.com/en-us/exchange/mailbox-migration/perform-g-suite-migration#create-a-migration-batch-in-office-365

Trigger the migration in the ECP (Exchange Control Panel)

Viewing the status of the migration

• Login to admin.microsoft.com
• On the left pane click Show All...
• Click Exchange under Admin centers
• Recipients => migration => Select the migration
• click View details...
• Select a mailbox
• and click "Download the report for this user"

Scan the logs and you might see this snippet repeated many times

Initial sync delayed due to configured constraint of 2 GB per 24 hours, will resume at: 11/3/2019 9:44:51 PM

The previous paragraph comes from the detailed logs of a stalled migration. So if you have multi gigabyte mailboxes to transfer then prepare for really quick transfers stalled by configured limits.

So what this means is when you start a migration you very quickly see mail appear in the user mailbox. But then it will stall for 24 hrs waiting for another 2GB of transfer quota and then the next day all your mailbox syncs will jump by 2GB and then stall.

I presume this is why it is good to pay a company that has an agreement with Google / Microsoft to by-pass these limits and quickly perform the migration

Calculate the time required for a mail migration.

Mail box size in GB divided by 2 = number of days to transfer

e.g 8GB / 2 = 4 days

Good over view here: https://techcommunity.microsoft.com/t5/Exchange-Team-Blog/Introducing-the-new-migration-experience-from-Google-G-Suite/ba-p/609375