It depends on how active video chat is, and all the other services that use RMS. THe thing to remember is that 2 people using video chat requires 2 concurrent video connections, both with an upstream link and a downstream link..... all passing through RMS. If those 2 users are using a fairly low resolution chat, each will require about a .5Mbps data rate, for a total of 1Mbps streaming data rate. At that rate, a server with a 100Mbps connection, will have that connection saturated if about 160 see more people are using video chat at the same time. All the rest of your normal site traffic like uploading and viewing photos will probably slow to a crawl.

How long you can get by with a RMS on a 100Mbit connection is entirely dependent on how users use your site. If video chat is a popular thing, you might need to upgrade to a 1Gbps server sooner than you think.