Apologies, have that backwards and did not have the API documentation or code in front of me at the time. I of course meant PONG.
[EDIT] Actually…
About once every five minutes, you will receive a PING :tmi.twitch.tv from the server, in order to ensure that your connection to the server is not prematurely terminated, you should reply with PONG :tmi.twitch.tv.
Are the IRC API docs wrong? I should expect a PING every ~5 minutes?