For the one CO character I put together, I already had a strong character concept, then used the suggested advancements for time in service in the PG.
The character ended up with 21 advancements, for a 36-year-old hotshot.
On the flip side, my Caitian engineer, Lt. Karlinarsti, started out as an enlisted character and has served for 28 years, giving him 40 advancements (Karl was heavily involved in the Tholian-Federation War).
So, I will echo the sentiments of others here in recommending that you get a concept for your character, and give him/her enough advancements to achieve those goals. For ease, I would suggest using the 1 per year in peaceful service, 2 per year in wartime, and 1 per 2 years of civilian professional life.
Davy Jones
"Frightened? My dear, you are looking at a man who has laughed in the face of death, sneered at doom, and chuckled at catastrophe! I was petrified."
-- The Wizard of Oz