EXPERT RESPONSE
It is difficult to point to any specific "rule of thumb" as capacity planning is very dependent upon the type of work being done at the remote site. For example, a retail location might require small Point of Sale (PoS) transactions whereas an office may require the transfer of bandwidth intensive graphics, real-time traffic (like VoIP) or other applications with unique handling requirements.
I would recommend profiling an existing remote location to get a baseline of the type of traffic you are seeing, noting the volumes of data that are sent during different periods of the day. If no such site exists, look at the traffic generated by a subset of users in the headquarters location and make an "educated guess."
It is often useful to use a WAN emulator to simulate different link sizes, latencies, and packet loss. This will help you predict performance under varying conditions. In addition, WAN optimization appliances can help save bandwidth and mitigate latency, which can save on costs by increasing WAN capacity (and improve application performance in the process). As these devices are being designed into WAN networks more and more, it is probably a good idea to include this into the evaluation process.
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