You'll know when it's time. You will see a change in her. You'll see it in her eyes.
I've been in this same situation a number of times. When you know her the way you do, you'll recognize when it's time. I don't envy you. These are the hardest times. Saying good bye to them is something I dread. I will keep you and her in our prayers.
Please keep us posted. If you can I would like to see these tumor location. It would be good for all of us to be able to recognize them. Best of luck