Each person is unique so there is no sure formula for when to talk about end of life issues but here are some ideas to consider ---
Everyone has the right to be told they can’t be fixed. What they do with that information is their choice. Most people will react the same way they have reacted to every other challenge in their life.
I find it sad when no one will be “real” with a person who is facing the end of their life. Most of us are uncomfortable talking about or even acknowledging decline and the evidence of approaching death.
The closeness of our relationship will determine how openly we discuss end of life with a person. If we are close to the person we need to look for an opportunity to have an open discussion. This is not a time for games that all is going to be okay, “we are going to beat this enemy” instead it is time to talk about love and memories.
If you are part of a family dealing with the challenge of a loved one dying you can, and should, begin a conversation with other family members at any point from diagnosis on. Bring out the possibilities and discuss them. Actually we should all have these kinds of conversations before we are faced with them.
If a casual friend, neighbor or church member is dealing with a life threatening illness you probably want to wait until they begin a conversation about their life situation. If they open the conversation door don’t hesitate to begin listening and offering information. Sometimes it is easier to talk about these matters with some one not as close to us as family.