How much internet bandwidth do I need to synchronize my documents with OneDrive for Business and Office 365?

To give you the right answer, that’s no so easy, there not only one factor:

• working day, where all employees work.
• number of documents, each user will generate each day.
• number of existing documents, each user will edit each day.
• average size of a document
• Average desktop and mobile Sync clients
• …
  The best method for determining bandwidth requirements when deploying OneDrive for Business is to measure the bandwidth used during a small rollout. The best way to ensure that measurement will be useful is to include a group of people that are representative of your organization and measure from initial deployment/sync through at least a week of edits/additions/removals.

To plan the bandwidth usage to work with Office 365 look here.

And since  10/15/14 there is an Excel based calculator to download direct form Microsoft Download Center.

[Update] Due to the next generation Sync client and the replacement of the old Groove client, Microsoft has withdrawn the calculator

Most difficulties for admins are, to get the correct answers from users to input the collected data. The calculator has two different user profiles.