Langdon is not a white-coin wonder: An Easter egg for those of us in recovery
In his final scene of the season, Langdon tells Robby he has been clean for 186 days. Earlier in the season, we are told his leave of absence has been for 10 months.
In addiction circles, it is common to relapse. For many of us, it's just an expected part of recovery. A "white coin" is the first sobriety coin you get to celebrate 24 hours of sobriety in many 12-step programs. Many of us have several of these due to initial relapses. (Depending on your program, you collect coins/chips of other colors and designs the longer you are sober. I myself have 137 days and keep a red coin in my pocket which I collected at the 3-month mark.)
A "white coin wonder" is someone who is able to stay clean in their first try. These folks are rare.
To state the obvious, the fact that Langdon has been "clean" for a shorter time than his leave of absence tells us he had one or more relapsed in the beginning of his recovery journey.
The writers this season have done a great job telling Langdon's recovery story. He's no saint (none of us are). This little bit of detail, while subtle, was a good addition.