downtime for cutover migration - office 365

We have an on prem Exchange Server 2007 and will be migrating to Office 365 online Exchange.  The following support article is what we are going by for the migration:

My question is how long would it typically take to migrate 100 mailboxes at approx 1GB each.  Also is there any downtime doing it this way?
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What down time are you referencing, you seem to be considering a one or the other.

An option is to transition with the user's for some time checking both for emails.

Does your current setup have archiving where a portion of the 1gb is in an archive?

Remote powershell access to office365 to ....
Make sure to setup the needed mail flow, trace rules before.
Once setup and tested, changes to MX will transition the senders from the old system to the new system.
al4629740Author Commented:
A portion for some users is in archive. yes.

I understand about MX records already....Correct me if I'm wrong, but if I follow the recommended steps, there should be no need for them to check both email boxes with no downtime  Have you done this before?  I want someone to verify that's the case or not.
Vasil Michev (MVP)Commented:
The cutover process will perform a periodic sync every 24h after the initial synchronization, which will bring up any messages that were delivered in the on-prem mailbox to the O365 one. So change the MX, make sure mail flows to O365, wait for the next sync and you can remove the cutover batch.
I've transitioned but not in your exact scenario, no on premises exchange. Hosted elsewhere.  Leaving the option of checking both for sometime will provide access.
That's 100GB of data. That will take a long time. You should test one account and see how long it takes. Even if you have a high speed internet connection, Microsoft's servers will not let you use the full bandwidth.
It may be a good opportunity to get the users to clean out their old mail before you migrate. Alternatively you could archive their mail to local PC's/servers/NAS using PST files.

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