There is no set length to each Advancement, AM. They vary according to the needs of the game and the character concept.
If you want a young but more skilled officer (say a Jim Kirk or Wesley Crusher), the advancements could represent as little as a few weeks each. If you want an older but less skilled character (like Picard after he "fixed" his past in "Tapestry"), the advancements would represent a much longer timeframe.
Former Decipher RPG Net Rep
"Doug, at the keyboard, his fingers bleeding" (with thanks to Moriarti)
In D&D3E, Abyssal is not the language of evil vacuum cleaners.