The time someone has left is based on the doctors treating the patient's opinion. This is usually determined by the illness and previous patients at the same level of illness.
For instance, my SIL was given 3 to 6 months by her doctor and then was given in home hospice care. The nurses treating her at home, because they had more first hand care experience, then told my MIL and other SIL she probably had 6 weeks-tops. It was at this point that my DH and sons went to NY to be with her. She survived six weeks from the date she was admitted to hospice care.
My mom's situation was the same diagnosis and chemo was started. If she had opted for it, she would have gone into hospice much sooner. Six weeks after diagnosis and beginning chemo, she was admitted to the hospital. She was in the hospital for a week, then she entered a hospice facility and passed 5 days later.
It's not exact. The hospice facility my mother was in (Woodside in St.Pete) had told us of patients who were there for a year, then it was determined that they were in remission and they left the facility. When patients enter Hospice, they (or their surrogate, which I was for my mom) agree that they will no longer pursue chemo or other life saving measures. Instead, they'll receive care to keep them comfortable and out of pain.
I guess it will always vary. The cancer that SIL and my mother both had is one that Hospice has a lot of experience with (small cell lung cancer), so they could accurately assess that they were within the six month guideline. My FIL lived a lot longer than anyone expected with the stage of emphysema he was in.
Suzanne