Q. How much bandwidth will NetFlow consume? How frequent is the traffic flow?
A. A quick answer is "not much". The switch/router summarizes the flow information, and typically will send an update about the flows it has seen every 60 or 120 seconds (this is configurable).
The rule of thumb is that NetFlow traffic adds about 2% to the total traffic. That's because every "flow" (session between two endpoints) can be summarized to a single "flow record"; thirty of these flow records fit into a single UDP packet.
Thus the NetFlow traffic is generally not a significant fraction of the total.
It is easy to set up your Cisco gear to send flow records, so you can see the effect on the traffic. We have written documents that can be found under InterMapper Knowledge Base.