How long does it take for a bone infection to turn septic?

Osteomyelitis (bone infection) can develop quickly or over time and takes a long time to treat with IV antibiotics, debridement, etc. If antibiotics no longer work the bacteria become resistant to the antibiotics and the infection gets worse.
In my experience amputation of the affected limb occurs at this point to prevent sepsis. If not amputated sepsis can and often does occur but it can’t be predicted when.
Best to speak with the doctor for specifics.

This is definitely a question to ask the doctor. In my experience as a nurse, once an infection goes to septicemia it takes one very quickly. This means that through the blood the entire body is septic, and one at a time major organ systems go into shutdown. However, there is not, in my own experience any way to tell how long something will go from bone to blood. Think of it this way; there has been the unhealing ulcer for 2 years or more, and that has not moved into systemic via the bloodstream. There is not any way to know. Do speak with the doctor, discuss what he feels a prognosis would be, what likely outcomes would be by this choice. I do not know if there is an infection control specialist here? I think there must be after all this time. This would be the best doc to discuss this with. I am assuming that you are medical power of attorney, OR your father has given permission that any medical information be shared with you. I am sorry you are both going through this.

This really is a question that can only be accurately answered by a medical professional familiar with your father's history and all his medical conditions.   W/o access to medical records, even the medical pros here could only give you a rough estimate.  

I would raise this issue with his oncologist, or other specialty physician.