An answer to find the probability that a number of x blocks to be found in a time frame in which you would expect to find λ blocks is found here :
bitcoin.stackexchange.com/questions/22978/how-can-we-be-sure-that-a-new-block-will-be-found/22984#22984
where x = 2 or more and λ = 1.
Any comment for this response is appreciated.
Thanks