Ebbie... every five years is considered the minimum here but you should talk with your GP first. There is a risk involved so there are a few factors to consider.

My old man was foreman of the jury (sequestered over two weeks) on a trial of two men charged with murdering (executing) two police officers in gruesome fashion after a botched kidnapping ransom drop/stakeout. http://www.cancrime.com/2009/05/23/the-double-cop-killing-that-shocked-the-nation/