You might want to look into what are known as Comet programming techniques to stream information down to your users, rather than having the client poll the server. This is actually a family of techniques, some of which may work better than others depending on the circumstances, such as what kind of server you're using and what kind of client compatibility you need.
If your server can handle a large number of open connections at a time (as in, it does not use an entire thread or process per connection, such as nginx or an erlang based server), you may wish to use a long polling technique, where as soon one message is received, the client immediately requests another message. If there are no messages available, the server simply keeps the connection open, possibly sending occasionally dummy data as a keepalive, until a message becomes available.
Comet, described by Brian is a nice technique, but requires session support on the server, which is probably more advanced than you care to implement for a simple chat box.
The best way to implement polling intervals is to imagine you having a chat window which you can minimize to do other stuff, or open to see if you have new messages. When you are in the middle of a conversation, you'll switch to it (poll) frequently. If you don't get any messages for a while, you will start looking rarer and rarer until you only check it occasionally.
Assuming you don't need to do real-time typing, you can probably poll every 3 seconds or so when at peak activity, and if nothing shows up for 5-10 polls, start to crank the interval up (perhaps doubling it every time) until it hits 30-60 seconds. Getting a message back should reset the poll interval back to a few seconds, while sending a message should poll instantly, but probably doesn't need to effect the frequency of polling otherwise.
Honestly, if you are implementing a “basic little AJAX shoutbox/chat”, things like Jabber, Comet etc are overkill for you. These things will require you to run additional
servers/proxies to take the load of the app server and db.
When you think about stuff like presence management (“Joe is typing...”), then things get overly complex for your app (considering “chat” is not your prime focus).
Think about adding widgets from providers like Meebo and Userplane. Once you scale think about the Jabber and the like…