How often do the RSS feeds refresh ?

From work I can't browse the forums as freely as I would want to, so I'm a big RSS user. I can see the newly created threads & it's easy to see the spam threads with it but I was wondering how often the feeds refresh.

In other words, how long does it take for a new thread to show up in the feed ? I'm using the personalised Google page (http://www.google.com/ig) & even when I refresh, it takes a while before new threads show up. I'm not sure if the "problem" is on Google's side or on TS so I was wondering what the refresh rate was set to.

We have various tasks set up that take place at different times, most of our XML feeds are updated 3 times per hour (including the forums, I believe news is the only one updating faster than that).