I ran CK3 with only this mod installed and then did 3 simulations of each different type of bubonic plague for a total of 9 (Minor, Major and Apocalyptic) by debug mode in the year 1178.

Peak(ed) = When the plague starts to break up and disappear.
Total = All provinces no longer have the plague.

Simulation 1, Minor: Total 2 years. Peaked at the 1 year mark.
Simulation 2, Minor: Total 2 years. Peaked at the 1 year mark.
Simulation 3, Minor: Total 3 years. Peaked at the 1.5 year mark. (This one was done far east purposely in a area of poor plague resistance to test the max cap out)

Simulation 4, Major: Total 3.5 years. Peaked at the 1.5 year mark.
Simulation 5, Major: Total 4.5 years. Peaked at the 2 year mark.
Simulation 6, Major: Total 8.5 years. Peaked at the 6 year mark. (This one was done far east purposely in a area of poor plague resistance to test the max cap out)

Simulation 7, Apocalyptic: 13 years. Peaked at the 10 year mark.
Simulation 8, Apocalyptic: 14 years. Peaked at the 11 year mark.
Simulation 9, Apocalyptic: 11.5 years. Peaked at the 8 year mark.

So, knowing how the Black Death normally spawns, it spawns as a apocalyptic plague. Since the mod makes the min and max province cap coverage 3x as much, it makes sense it'll hit the poor regions and the poor regions get constantly reinfected/reinfect others around them. That's why it's lasting so long. At a certain point, people get resistance to the plague and it'll start breaking up. So the mod seems to be working fine, apocalyptic is really hell when Black Death appears, but that's intended.