I believe 95% of pilots are in their first base preference, and a good proportion of those who aren't are those that have left small, popular bases (EXT/BOH/BRS) for the command upgrade.
Time to command is hard to gauge. The latest upgrades are around 11-12 years in the company, however there was a six year gap in recruitment between 2007-13, so one assumes that time is going to come down significantly very shortly.