Every fan fiction writer I've ever met has wanted the crew of their ship(s) to stand out in some way—to be so special that they stick in the reader's mind, making them want to read more. I admit that I'm no different, though for the most part I have done my best to have my characters advance through the ranks at a realistic pace.
To give you, the reader, the same idea as to why someone who is 35 has just made lieutenant commander when Kathryn Janeway made captain at that age—and James Kirk did it even younger—here are the tables I use to determine when each character (save for those few special cases) has achieved each rank. For the purposes of my fanfic universe, I've replaced the rank of rear admiral, lower half with commodore.
The officers table was given to me by a fellow writer. The enlisted table I made myself after researching the associated time-in-rank for enlisted personnel.