My wife and I just finished the DCF process in Tokyo. It would have taken about 3 months, but we had to delay the final interview for about one month since my wife needed to travel outside of Japan and we didn't want her to have her interview until she had returned from that trip.
As for the cover letter, I made sure to describe my wife and I's relationship, the nature of the job offer I had received, and the importance of the job for the success of my career and my professional relationships, and how long term separation from my wife would negatively impact our lives.
EDIT: added some missing specifics