What are the criteria for advancing from Ensign to Lieutenant junior grade in Starfleet?

Question

The Memory Alpha Wiki says:

At this rank [LTjg], [officers] had begun to prove their ability to perform acceptably in Starfleet, and typically began to take on more senior roles in command situations.

But what actually qualifies an ensign (or an enlisted person etc.) to be appointed/promoted to a Lieutenant junior grade? What are the sufficient and necessary conditions, materially and procedurally?

From the same Wiki page we know that one sufficient condition is completing one's medical studies and training as a doctor; so I'm mostly interested in ensigns.

Note: If time is significant, let's make it TNG-era.

Answer 1

Since we witness very few promotions within Star Trek, we don't have a lot of in-universe information to work on.

The show also has some oddities - there seem to be very few non-commissioned officers, with ensigns and lieutenants apparently covering what would be the job of a chief at times. We do see the odd crewman from time to time, but Starfleet seems to be mostly officers.

The EU muddies things even more, because writers want to write about Commander Nog and Captain Ezri Dax, and generally don't understand the timescales or pathways (like command school) needed.

The TNG episode "Lower Decks" is the only episode that really gives us insight to the Starfleet process. It shows a number of ensigns being considered for promotion. This seems to be treated as simply as a staff evaluation would be in a large business - every few months; nominated by supervisor (Worf or Geordi); evaluated by Human Resources (Riker and Troi); signed off by the boss (Picard). Other than good performance in current duties, there doesn't seem to be any further testing required to advance. This actually isn't all that different from the actual military in most countries.