I think a more interesting question is, how big is the probability of not finding a block within time period in a mining pool of x% market share.

Knowing this enables you to audit pools.

Example:

Slush could not to mine a single block for more than 2 days between 19 and 21. Jun 2015, see
https://mining.bitcoin.cz/stats/blocks/?page=23

Slush' market share was 2.2% around that time, see http://organofcorti.blogspot.hu/2015/06/june-14th-2015-block-maker-statistics.html

Such a bad luck has a probability of only 0.17%