Sadly that is not correct. Leukemia is a cancer where the white blood cells are mutating in the developmental stage in the bonemarrow. That means that the production rate of white blood cells are a lot quicker than they should be and block the space in the bone marrow for other cell production such as red blood cells and platelets. They also escape into the blood stream. So there is really no good or bad in the sense that not treating it will definitely lead to death. This does not take very long. The one thing that differs between the different sorts of leukemias I listed is how they respond to treatment.

Leukemia is diagnosed by studying the cells in the bone marrow and seeing at what stage the mutation begins in the white blood cell. Children (and yes 15 counts as a child) will spend a minimum of 2 weeks in hospital from the moment they are diagnosed to treat the initial leukemia and from there on out spend a minimum of 1 year in and out of hospital because of chemotherapy and eventually infections due to not having an immunesystem. These visits would last between 3 days tp 2 weeks. She would have lost all of her hair as the chemos are strong AF, basically raw poison, and she would be very malnurished. She wouldn't be able to talk at certain points because her mucous membrane would be fried (lots of ulcers and sores in her mouth). She would not be able to go to school as the infection risk is too high but instead attend school at the hospital or be home schooled. She would be in constant aggony.

I would seriously first figure out if she really has cancer. It could be that she is in palliative care, but she would require regular blood and platelets transfusions as her own bone marrow will not be able to produce enough. Ask her if she has a hickmann line or a port-a-cath. These are ways for her to receive her medication, blood transfusions and have her blood drawn.