mine is just a month old php site with yahoo group message archive. The application updates the sitemap as soon as gets the messages from yahoo groups. And it will ping google once it is done like this http://www.google.com/webmasters/sitemaps/ping?sitemap=http://your-site/sitemap.xml You can create static urls in a seperate xml file and dynamic ones are in diffeent one and create sitemap index file.
170 or so directory sites. I submit them on average every two weeks, in rotation, simply because that's how long it takes me to get through the list. Any site that has signifigant changes gets submitted immediately, regardless of schedule.