Can screen-scraper be scheduled to scrape sites on a periodic basis?

If you're using the Enterprise Edition of screen-scraper, this can be done via the web interface.

For the Basic and Professional editions, the best way to go about this is to use an external scheduler, such as the Windows Task Scheduler or the Unix cron daemon. You'll typically set up one of these schedulers to either invoke screen-scraper from the command line or to invoke a separate application, which in turn invokes screen-scraper while it's running as a server.