Well, assuming the last 2 cards are not AA, the answer is...
(4/52)*(3/51)*(48/50)*(47/49)*6 = 0.0249995 = 2.5%, basically 1 every 40
hands. Contrast this with texas hold'em where you get 1 every 271 on average.
If you take any AA (which includes AAAx and AAAA), the answer is...
(4/52)*(3/51)*6 = 2.7%.