I am taking an online class through the University of Nicosia in digital currencies and covers Bitcoin and the Blockchain. It is a free MOOC and can also be used as credit towards their Masters in Computer Science.
During the Live Q&A section on October 3rd, 2016, there was a question about the timing between blocks being mined. It was suggested that we look at the previous 2016 blocks to see what the average time between blocks mined.
The Bitcoin protocol is set to average the time to be 10 minutes between blocks. Every 2016 blocks the network readjusts the difficulty so the average time blocks are mined is 10 minutes. If the average time is longer than 10 minutes then the difficulty decreases, if the average time is shorter than 10 minutes the difficulty increases.
The average time to mine a block was 563 seconds or 9:23. The longest time was 1:17:36 and the shortest was less than a second.
Because the average time to mine was less than 10 minutes, the difficulty was increased starting at block 431424 to 241,227,200,229.99