The answer that Mike Pennington gave is the best 'real world' answer, but Cisco documentation lists ~1.5mbps symmetric. I think they assume you to have a private lease line and use traffic engineering to deprioritize or drop non-business critical data. In this day and age, you can't really get by without people using streaming internet services - so know that his answer is the more accurate one, in that regard.
I think a good approach for psuedo-realistic accuracy, is to enable NetFlow/Flexible NetFlow on your current equipment (Router or Switch, if supported) and export it to a NetFlow collector. Further, use of IP SLA could also help you in capacity planning, but will not be as effective or fruitful as NetFlow.
I can't tell by your question if you're setting up a new office or migrating an existing office to a new connection. However, assuming this is a new office and you can't get a baseline. You could use NetFlow on your current corporate network and divide out the user population into percentage of segment use (or percentage of VLAN use) and you'll end up with a close enough estimate to baseline. Look at your new office layout and access requirements. This will give you a minimum value.
By collecting NetFlow data, you can see when your peaks are occurring, what kind of traffic it is, and watch overall conversations inside and outside the network. Some additional things to keep in mind, is expansion. Don't order circuits that are needed today, when you're getting X new hires over the next year, or couple years even. It may be more expensive up front, but this is why you need to think about needs over time.
Lastly, don't overlook the usefulness of QoS. You CAN draw blood from a stone, especially if you have a mix of critical and non-critical data. Instead of making the entire pipe bigger, you can give first access to those critical services, and either drop or slow down the non-crit traffic.
As a final aside, if this is a big issue - look into a traffic analyzer if netflow isn't supported on your equipment. Setup a SPAN port, as appropriate, and let it watch your network activity. Make conclusions from there.
Here's what Cisco is saying about capacity planning, but don't expect to draw anything other than conceptual knowledge: