System.Net Network Stream

Hey I have a question about streams.

I'm working on an application that will connect to a chat server, as the server will be constantly sending data to the client that I'm building, I was planning on useing a network stream to read the data from the server. All this data will be stored into the buffer, be converted to string and all the good stuff.

My question is:
I read about the network stream class and it said that networkstream will continue to read until the buffer gets full. If i'm going to be connected to the server for hours upon end, and receive alot of data, how do I keep my buffer from getting full?

I have built my string builder to convert server data, but im not sure if it's clearing the space used in the buffer when it's converted. I hope im being clear enough in my question.

Right now it reads from the network stream, converts it to string and prints it in a text box. Do i have to build some sort of disposal method, or flush the buffer? I haven't used flush before, so I'm not too sure about it. Any ideas?