Arthur Morgan finding out he that has tuberculosis is one of the most heartbreaking moments in all of Red Dead Redemption 2. The illness forces Arthur to finally reflect on all the awful things he has done in life, and acts as a turning point for the story as a whole. With how important Arthur's disease is to the Red Dead Redemption franchise, it is worth understanding exactly how long he had been sick for.
Towards the final portion of Red Dead Redemption 2 players can trigger the mission "A Fork in The Road," which only occurs after Arthur es out in Saint Denis. A man will help get him to a doctor, who will briefly evaluate him before eventually diagnosing him with tuberculosis. Instead of providing hope, the doctor expresses sympathy in what is essentially a death sentence, leaving Arthur to reflect on both his future and his past. This mission may be where both the player and Arthur learn about the disease, but considering he can be heard coughing beforehand, it is not the exact moment he "became" sick.
During Red Dead Redemption 2's Chapter 2 mission "Money Lending and Other Sins" Leopold Strauss asks Arthur to travel to the Downes family ranch and confront Thomas Downes about his debt. During the mission Arthur gets into an altercation with Downes, which results in the latter coughing up blood on Arthur's face. Since Arthur later finds out that Downes succumbed to tuberculosis, it is easy to identify this as the exact time when Arthur was exposed.
When Arthur Actually Developed Tuberculosis
According to the official CDC website, people who are exposed to tuberculosis can develop the disease as quickly as within a few weeks. However, in some cases it the disease may not surface for years. Since Red Dead Redemption 2 predominantly takes place 1899, it can not possibly have taken over a year for Arthur to develop tuberculosis.
A reasonable estimate for the total amount of time that Arthur was sick in RDR2 is around 3-6 months. This would have have allowed enough time for events that preceded Arthur's diagnosis but followed his interaction with Downes, such as the gang's activity in Rhodes and time in Saint Denis. This window also s for the time that ed after Arthur's diagnosis, although by this point the pace of Red Dead Redemption 2's storyline increases significantly.
Arthur's illness is one of the main things that forces him to seek redemption in RDR2, and living with the burden for so long is what shapes him into the character he becomes. Unlike many of Red Dead Redemption 2 most notable deaths, which come as a surprise to players, Arthur's is drawn out, allowing players to watch the protagonist try to change into a good man during his final days.
Source: Official CDC Website
