Writing a Military Scifi Book

[Apache]

I wanna keep some of my facts and stuff straight.  I was hoping you guys could help me out.  I gotta lot of questions mind you! 

Here's a Marine Corps question.  How long has a Colonel typically been in the Corps?   Same question for a Captain in the navy.  Especially if they were pilots at one point.
 

[Collo]

It all depends on several different factors; such as service history (deployments etc), skills and education qualifications, position available etc.

In my book it would be reasonable to be a Colonel/Captain after 20 (at the earliest) and 25 years service.