2
The feeds update on:
1) page hits
OR
2) chron jobs
OR
3) when you hit refresh in module admin
Everytime the page is viewed (or your cache settings) determine how often the information is updated, you can also use an automated process (chron or the likes) to schedule an automated refresh of the page on a periodic basis.
Check out
http://peak.ne.jp/xoops for more info on xhld. in general.
You can google chron (or cron) or if hosted, crawl around your control panel or check with your host to see what type of automated scheduling services they provide.