It is sometimes stated that the typical law dean serves 2-3 years before stepping down. This is almost certainly inaccurate and may result from a misreading of the great data that Jim Rosenblatt collects. The median length of service of current sitting deans is 3 years. This does not mean, though, that this is the typical length of time a dean has served by the end of his or her term.
One way to look at this is that if every dean now serving suddenly resigned, their median length of service would be 3 years. But, despite the wishes of those who might see mass resignation as a good thing, most currently serving deans will continue to serve next year and probably longer.
Most years, there are 30-40 new deans appointed. This translates to 1/5 or 1/6 of all schools having a new dean. Therefore, the median tenure of retiring deans is probably in the 5-6 year range. Whether that median is declining is unclear, although that would not be surprising in these tough times for law schools.