Imagine a pain scale of 1-10; pretend 10 is so high that you die from the pain.
Is it worth living at a 7-8 at every moment continuously, to extend your life for 2 months? How about living at a 9 for just 2 more weeks?
If you say "no" that just means you're not hooked up to a machine where that's your living experience (not like someone is actively killing you).
I think, when you put it that way, it becomes easier to set some rules around it. Btw I'm pretty close to someone who would always choose to live longer, but even I would say no to, for example, living at a 10 of pain for just 1 more week of life.