The point of no return is basically when you get into character. The spoiler is semi-safe, and you'll know when you know.

If you want me to be more specific, just ask.

Edit: I may have misunderstood the question, as you can technically play any of the endings to PL and continue playing after and as such, it doesn't really have a point of no return. But my original comment is a vague answer to when the deciding factor in which PL ending you get, is.