How long can someone survive once their organs shut down from cancer?

It would depend on the cancer. If the cancer is in the liver and it's liver failure a person could last a few days. If it's lung cancer the body can't get enough oxygen to everything so the heart basically beats itself to death trying to get oxygen to all the organs but again that could last for several days. And, there are drugs they use to treat organ failure so you already answered the question, it's up to the good Lord. God Bless.

Um...they don't.

This is a pretty open question and I will try to answer it the best I can. Cancer hurts because of where it is. Bone CA hurts because the bones have tumors on them that shouldn't be there. Lung CA hurts because it causes you to not be able to breath. And so on. Where ever the CA is it will hurt because it is interfering with the body's natural process. Cancer is just a cell that has made a copy and that copy was messed up some how. Then the messed up cell makes a copy and so on, and soon you have a whole bunch of messed up cells floating around. Now everyone makes messed up copies now and then but usually your body gets rid of them (your immune system attacks them) before they make other copies, so there is no problem. But if your body makes a bad copy and it makes a copy before you can fight it off-this is cancer. It only affects other organs if the cancer spreads. People usually die from the complications associated with the type of cancer. Like lung cancer- they have problems breathing. I hope this helped, and I am sorry that you have to experience it in your family.

which part of organ?

I'm so sorry to hear about your sister. It depends on how sick she is already. It could be a few days, but with the good Lord, you never know. I just pray that she is comfortable. Look at the Hospice website, it may help you with what you are going through. Also, you can find ways to make her more comfortable.
http://www.hospicefoundation.org/endOfLifeInfo/

God Bless