On question I get from customers fairly often is how long it will take to migrate my mailboxes to Exchange Online from my on-premises Exchange organization with a Hybrid Exchange environment. As a consultant, I am required to answer “It depends”.
When you do the move
Exchange Online is a shared tenant environment. This means that your mailbox is likely located on servers that are shared with other customers. In order to ensure that all mailboxes on your server(s) have enough resources to function properly, mailbox moves into (and out of) Exchange Online are throttled. This means that if another Exchange Online customer is moving mailboxes into Exchange Online at the same time you are, you might have to “get in line”. There are times when people are more likely to be migrating mailboxes into Exchange Online, and thus you’ll be more likely to be queued. I find that Friday to Monday is when most people prefer to migrate mailboxes into Exchange Online, so that is when you’re more likely to be queued. Moving your mailboxes Tuesday to Thursday will get the job done sooner.
Version of Exchange hosting mailbox
The server hosting your Exchange on-premises mailbox can be Exchange 2003 through 2013, and I find that mailbox moves go faster the newer the version of Exchange hosting your mailbox. Additionally, mailbox moves from Exchange 2003 force the user to log out of their mailbox and this can make even short mailbox moves much less tolerable. If the mailbox you are migrating to Exchange Online is homed on an Exchange 2003 server, you might consider moving it to your Hybrid server (which would be Exchange 2010 in this case) before migrating it to Exchange Online. This will give you a much more predictable outage time frame while the mailbox is moved between Exchange servers on-premises, and then allow you to move the mailbox to Exchange Online without affecting the user’s ability to access her mailbox.
Exchange 2003 servers also require offline defragmentation, something that many companies don’t do or don’t do often enough. I find that mailboxes migration from Exchange 2003 servers that have recently had offline database maintenance run tend to go faster.
Version of Exchange running Hybrid server
I find that the version of Exchange used for the Hybrid server can make a difference as well, but not as much as the version running your mailbox server.
Troubleshooting slow mailbox moves
If at this point you still think there is there a problem with your mailbox moves into Exchange Online, Microsoft has provided a tool you can use to test your connection to Office 365.
This is a Java based tool that will run a series of tests from your computer (best not to run this directly from your Exchange server since it require Java) to Office 365. It gives a good report containing quite a lot of information. You can see that my consistency of service rating was low when I ran this test.