Hello,
as discussed in this topic:
http://www.xml-sitemaps.com/forum/index.php/topic,124.0.html,
there are the following options in this case:
- increase max_execution_time setting in php.ini file at your server and restart apache
- execute sitemap generator from command line (if you have ssh access to your server)
- use "Save state" option at Sitemap Generator configuration page and execute the crawler multiple times with "Resume generation" enabled until full sitemap is created
- limit the number of pages to index in Sitemap Generator configuration