I don't think suicide can be rational simply because you cannot know the future. That said, I also think if someone decides their suffering exceeds their tolerance, that is a determination only they can make, because they're the one who has to live their lives. I would never fault someone for doing it, and I believe everyone should have the choice... but by definition that choice cannot be rational.
Even in the worst possible conditions of life (extreme torture, enslavement, terminal illness, hopelessness), something unforeseeable might change about your situation or perspective at any moment. So there's never going to be a point where you can definitively say "ah, yes, I know what's next, and I've determined it's not worth going forward"—and have that be factually correct. You just have to decide you're okay with living in the uncertainty, while things are terrible now. If you can't, then, that is your choice to make... and while I can't ever truly say it's the "right" choice, I can say that I understand.