How often are these pages updated?
The short answer is: it depends (what you mean?). :-)
Based on wanna-build dump from buildd.debian.org, generated at: 2005-09-09 14:43:16 UTC+1
There you can see, when the data behind that page is last updated or - more exactly - last obtained from it's data source, usually a wanna-build. This timestamp is also listed when you do a Package status search: beneath each arch name, the appropriate timestamp is listed.
The HTML pages and the host & buildd status itself is updated every 5 minutes. So, when a buildd updates its status it takes at most 5 minutes until you can see the changes.