I see that new questions appear for a certain tag, but there is no RSS feed for that. How often is the feed updated or how long must one wait for RSS to send a notification?
If you are using Google Reader you can hover your mouse over the date and it will tell you the time the post was originally posted and the time it hit reader. I checked a few RSS feeds from SO and the delay ranges from 20-60 minutes. That's about as reliable as they get from past experience.
If you're looking for a feed that can be refreshed once per minute and contain up-to-date information, I recommend a small tool I created called stack2rss.
It converts API requests into RSS feeds that can then be subscribed to.
An example that displays all new questions on ServerFault tagged with