I have say, one of the top five questions I am asked by enterprise customers who are coming to the cloud is “how will my bandwidth change”? For large organizations this can be a complex discussion on understanding how the organization accesses the Internet today and how they plan to change with time with the introduction of devices. With many of the organizations I work with, they have very specific controls to ensure that all inbound and outbound traffic is routed through their networks so they can properly inspect the traffic.
With all that said, there was a recent blog created called “Tune and optimize performance of your Office 365 connection” - http://blogs.office.com/2014/10/29/tune-optimize-performance-office-365-connection/. There some good information here if you are trying to figure out what is the bandwidth impact of Office 365 to your organization.
Network Planning and Performance Tuning reference - https://technet.microsoft.com/library/f97c2f06-0426-443d-8a16-d98abb0da252. This TechNet Article has been around for a long time and is updated on a regular basis. I highly recommend organizations read this as part of their evaluation when going to Office 365. Additionally there are calculators for Exchange, Lync and OneDrive for Business that you should review. There are network tools to analyze traffic, along with best practices and network planning activities.
Finally there is a new Microsoft Virtual Academy on Office 365 Performance Management located here - http://www.microsoftvirtualacademy.com/training-courses/office-365-performance-management. There are numerous modules in here with some really good facts. I really liked Module 7 which was a session on best practices on what some consultants have seen. In it they said there is no perfect number that can be determined before a deployment, however the most impact that they have seen is upwards to 20% new traffic and based on your Internet connection methods remote offices will be most impacted. Additionally I liked Module 8 on Configuring Firewall Whitelisting Planning because this is something that comes up a lot with customers whom I work with.