this error message means that your server is configured to interrupt scripts that are running longer than defined (30 seconds) value, but sitemap generator requires more time in your case to complete crawling of your site.
You can do either of the following:
1. Increase max_execution_time setting in php.ni file (if you have access to it) and restart apache - that will allow script to run longer
2. If you have SSH access toyour server, you can execute sitemap generator from command line - the time limit will not be applied by a server in this case
3. Exclude certain URLs from adding to sitemap using "Do not parse URLs" and "Exclude URLs" option - it may be enough time to crawl a part of site then.