This is a php.ini setting - http://uk3.php.net/manual/en/info.configuration.php#ini.max-execution-time

As default max_execution_time is set to 30 in php.ini

Therefore for some reason whatever you are doing is taking longer than 30 seconds to execute.

Rather than changing this figure, I would investigate to see if you have added something to threads to affect this, or if there is a server issue that is affecting this.