There is An Overview and Discussion of Various DNA Mutation Rates at the the kerchner.com site that notes that DNA mutation rates vary between markers and between families.

It reports that early studies used an average rate of one per 500 generations or a rate of 0.002. A recent FTDNA study is said to indicate an average rate of 0.004.

Using the average mutation rate for random changes, some estimates can be made of a match depending on the number of markers tested.

For 12 markers no change could be expected over several dozen generations. This means matches can be common between people with different surnames.

For 37 markers it is a theoretical estimate of no change for 12 generations. So a match is a good indicator of a common ancestor within the time frame of usage of surnames. There still is the probability of a common ancestor much further back in time or a random combination of markers from very ancient times.

Data is being collected at the kerchner.com site on mutations with an Mutation Rate Log Analysis. There are more than 50 surname groups with separate data for 12, 25, 37 markers. For calculations based on father-son mutations in 37 markers cases the average of mutations so far is 0.0043 or 1/232 generations. The fastest rate was 0.009 or 1/111 generations