What is the average time that passes between when the questions are posted and the answers which get accepted are posted? (not until the answer is accepted, but until the answer that got accepted was posted)
The average is 3.71 days... but the standard deviation is 25.2 days.
It number is so high because there are some questions like Date arithmetic in Unix shell scripts that take over 2 years to get the answer, creating a strong bias towards a long time. See this query for the top 100.
The median is more sensible — 0.0147 days, which is 21 minutes.