The fixes tag contains the commit SHA of the commit that was fixed, so one can look up the date of the fix and of the commit being fixed and determine the time taken to fix a bug.
Out of the thousands of "Fixes" tagged commits and the time to fix an issue one can determine how long it takes to fix a specific percentage of the bugs:
Comparing the time to fix issues for kernel versions v4.0, v4.10 and v4.20 for bugs that are fixed in less than 50 days we have:
... the trends are similar, however it is worth noting that more bugs are getting found and fixed a little faster in v4.10 and v4.20 than v4.0. It will be interesting to see how these trends develop over the next few years.