I'm not a sysadmin, I use a managed hosting provider for my instance, so I don't have expertise on bandwidth. I haven't had any specific bandwidth restrictions or estimations mentioned by my provider.
The amount that p2p helps depends on whether people are viewing the same version of the same video at the same time. If they are, then the p2p system helps out, and you'll see this with the "peers" number going up on the video itself.