Last pole shift was actually about 780,000 years ago. On avg pole shifts occur about every 200,000 years. They have plenty of evidence to suggest that pole shifts when they do happen, happen extremely quickly we are talking a matter of hours possibly. The damage to life on earth during the time the magnetosphere is down would most likely be very minimal as far as cosmic radiation is concerned. The earthquakes, super volcanoes and tsunamis are another matter.