From physorg, a statistical analysis of the velocity of evolution from the fossil record, looking for an upper limit:
Large evolutionary changes in body size take a very long time. A mouse-to-elephant size change would take at least 24 million generations based on the maximum speed of evolution in the fossil record, according to the work of Alistair Evans and co-authors. Becoming smaller can happen much faster than becoming bigger: the evolution of pygmy elephants took 10 times fewer generations than the equivalent sheep-to-elephant size change.
Now that computational cycles are becoming cheaper, some really interesting statistical analysis can be done in science to ask questions we never could before. Very cool.