Assuming a normal distribution, one would expect a 7-sigma occurrence once every 3,105,395,365 years.

Using Excel and MATLAB, a paper from UCD Business Schools calculated the probability of up to 25-sigma. Needless to say, the calculations are tongue-in-cheek given they quickly become meaninglessly large.

http://www.smurfitschool.ie/academicsampresearch/workingpapers/wp_08_13…