How often does poles on Earth change their polarity?

readability="4.8795180722892">

src="http://d2hej51cni6o0x.cloudfront.net/images/user_icons/Educator_Female_Small.png"
class="profilePic" alt="wannam's profile pic"/>
src="http://d2hej51cni6o0x.cloudfront.net/images/core/educator-indicator_thumb.png"
class="premium-indicator"/>

The time between
the shifts in polarity varies and we're not exactly sure why. It can be as long as 40
million years between shifts or as short as a few hundred years. The last polarity shift
was about 780,000 years ago. Some scientists think the polarity will shift again in the
year 2012. That is part of the basis for some of the doomsday prophecies about the end
of the world coming next year. We know that some planets have had catastrophic events
that could have involved polarity shifts; however, it is unlikely that the Earth will
suffer much from these changes. We know that the polarity has flipped before, so it is
unlikely to be the end of the world as some claim. Although you could average together
the times the polarity has shifted, it wouldn't give you a very accurate idea of the
process. The polarity changes occur at such different time intervals that an average
really wouldn't be helpful in this situation.

class="message-footer">

data-id="342853" data-method="/api/2.0/qa/rateAnswer">
like

id="upvoteCount-rateAnswer-342853">0


data-method="/api/2.0/qa/rateAnswer">
dislike
itemprop="downvoteCount"
id="downvoteCount-rateAnswer-342853">0