For reference, here is how many days/hours the system is unavailable with different percentages:

  90%      36.5 days ("one nine")
  95%      18.25 days
  98%      7.30 days
  99%      3.65 days ("two nines")
  99.5%	   1.83 days
  99.8%	   17.52 hours
  99.9%    8.76 hours ("three nines")
  99.95%   4.38 hours
  99.99%   52.56 minutes ("four nines")
  99.999%  5.26 minutes ("five nines")
  99.9999% 31.5 seconds ("six nines")

http://en.wikipedia.org/wiki/High_availability#Percentage_ca...