Buildd.Net

The short answer is: it depends (what you mean?). :-)
The long answer is:
The data behind these pages is updated every hour currently for unstable and every 12 hours for experimental, non-free, volatile. On every page there is something like:

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.
Of course, when the demand is high for it, the updates can be done more frequent.