We have modules and products to help with that assessment:
UC Assesor Path Insight Network Manager module in Prognosis (Network Central display etc) - This is an infrastructure type metric and will let you know what traffic is hitting the monitoring node NIC.
Every environment is different so it is hard to give 1 figure that would cover all environments. That's why we recommend these extra products. The first two will need extra licensing and you should discuss your exact need with your account manager first so he can recommend and sell you the appropriate licenses.
I believe most license do give access to Network Central in Prognosis, so maybe start with that one.