The random chance is very very low. For simplicity, let's say that 50% of the hashing power agree and 50% of the hashing power disagree. Then the probability of getting 750 blocks is 7*10^-59 ( http://www.wolframalpha.com/input/?i=binomial+distribution+n... ) There are approximately 50000 blocks in a year, so the expected time is a lot of millions of years.
This changes when most of the hash power agree. If the 74% agree then the chances are 24% ( http://www.wolframalpha.com/input/?i=binomial+distribution+n... ). The calculation is more complicated because the intervals may overlap, but this would take less than a year.
So, when more than 70% of the hash power agree, it's time to upgrade, but 70% is a clear majority.
(Just for curiosity: With 75% hash power you get a 52% chance. With 76% hash power you get a 78% chance.)
This changes when most of the hash power agree. If the 74% agree then the chances are 24% ( http://www.wolframalpha.com/input/?i=binomial+distribution+n... ). The calculation is more complicated because the intervals may overlap, but this would take less than a year.
So, when more than 70% of the hash power agree, it's time to upgrade, but 70% is a clear majority.
(Just for curiosity: With 75% hash power you get a 52% chance. With 76% hash power you get a 78% chance.)