The amount that you need "without interfering with other data" will all depend on the actual traffic on your network. I'd highly recommend that you look in to the traffic that is already present on your network and then determine what may be available for you to use with your video surveillance. The "percentages" in your network will be different to others, as you will have different traffic and requirements.
If you set up and use a proper QoS design throughout your network, then you will be able to appropriately tag and prioritise the traffic types to ensure all the critical traffic passes.
Wayne
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