You can change the default timeout value (600 seconds) in iChat with the following code in the Terminal:
defaults write com.apple.ichatagent UserIdleTimeout x
where x is the new value, in seconds. Note: you'll need to quit and relaunch the iChatagent background application for this to take effect (which is *not* done by quitting and restarting iChat -- you'll need to use the Terminal or Activity Monitor to find and kill this faceless process).
Mac OS X Hints
http://hints.macworld.com/article.php?story=200402181751007