http://www.Strategypage.com, in remarking about the recent spate of CO firings in the Navy, notes the following about the surface warfare officer career path:
It's a hard slog for a new ensign (officer rank O-1) to make it to a ship command. For every hundred ensigns entering service, about 90 will stay and make it to O-4 (Lieutenant Commander), usually after about nine years of service. About 67 of those ensigns will eventually get to serve as XO (executive officer, the number two officer on a ship) after 10-12 years of service. Some 69 of those ensigns will make it to O-5 (Commander), where it first becomes possible to command a ship (a frigate or destroyer.) About 38 of those hundred ensigns will get such a command, usually after 18-20 years of service, and for about 18 months. About 22 of those ensigns will make it to O-6 (Captain) after 20-21 years of service. But only 11 of those ensigns (now captains) will get a major seagoing command (cruiser, destroyer squadron). Officers who do well commanding a ship will often get to do it two or three times before they retire after about 30 years of service.
This varies dramatically with the aviation community, wherein an officer has an approximately 20/100 chance of making CDR from his initial group of ensigns, and just a 1/100 chance of command from within that same group.