I refresh the 'Newest' tab of a particular tag every so often throughout the day to keep on top of new questions asked. However the caching used on Stack Overflow and presumably the other sites seems quite aggressive. I'll refresh and find a question asked 30-45 minutes ago that wasn't showing up when I last checked about 10 minutes ago (note: accurate testing not performed!).
What determines expiry of the cache and what is a good refresh interval to use?