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The timeline for any organization's Office 365 deployment is dependent on the situation. The parts of Office 365...
that require the most time are mailbox and content migrations. Most other services -- such as Teams, Groups, Skype for Business and Office client -- have fewer legacy connects and merely require configuration or deployment. The remaining features are usually just enabling and change management.
When trying to calculate mailbox and content migrations, you should take the following into consideration:
- number of mailboxes;
- mailbox source -- e.g., Exchange, Notes and GroupWise;
- existing content volume;
- available network bandwidth;
- organizational complexity -- e.g., many disconnected departments or groups;
- employee geographic distribution;
- migration tools;
- SharePoint site complexity; and
- content volume in file shares.
Mailbox migrations during the Office 365 deployment can be immediately throttled by Microsoft to about 1 Gb per user, per hour. However, you should expect roughly half of that throughput. Compounding throughput challenges will be your available bandwidth at each location.
Secondarily, depending on whether you're migrating Exchange on premises, Lotus Notes, GroupWise or some other system, you may run into other complexities related to whether the source can run in hybrid mode, where your on-premises environment coexists with Office 365, as well as what migration tooling is required.
Content migrations are also governed by migration tooling; where the content will live, such as SharePoint versus OneDrive; and your taxonomy complexity -- e.g., whether you have metadata migrated, just content or a combination.
In all, you should anticipate a six- to nine-month migration window for an Office 365 deployment for most employee populations -- up to about 50,000 employees. This timing includes basic organizational change management, which tends to be a critical factor in any organization's move to Office 365. Timing will be shorter with smaller firms and longer with larger or more highly complex environments.