At least 2 months? Probably even longer. I say 2 months because that's how long the plague lasted, and then probably even more time because of the construction of the medicine factory and earning a spot as a teacher at uni. Also, I think Farma mentions he's been in the other world for a little more than a year, which surpasses the usual time "gods" in a human body stay in that universe.
Chapter 20 did mention palle having bruises and how fast he got sick so that's how early we got
But palle has been training them harshly for quite some time even before Farma got here so he could die without worry, so I can only assume it was since the harsh training started with the original Farma that that's how long he had it
Was it ever stated how long Palle had leukemia for? We have a time frame of the first hint (chapter 20)
I'm just asking if he had it before or after MC got here and for how long
If it was long before then it'd kinda feel like god was answering his prayer and devotion