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.