While I think a sense of timing may be needed for emotionally vulnerable people, I think that it is always the right time for truth. I don't really see this as a gradient. I don't even know if I understand the question. Why would I withhold the truth from someone unless I was trying to hide something from them? I don't understand "games" like this.
However, I think, perhaps, the essence which you speak of is the "right" time for realization. My dad had been battling cancer for 5 years when, in February of the year in question, he started to fall ill and had increased hospital visits. He had his gall bladder removed, had many infections which were life threatening, and, by May, was in near-debilitating pain.
I told him that I hoped they were going to be able to manage this and get him to a new normal. At this point, he sort of spaced out, the way he did when he was attempting to keep his emotions to himself, and said, "I don't know about that this time." I understood then that he was coming to terms with his death and he was more than just prepared as I had seen him up until that moment, but more that he was ready.
He got very sick the following June, and my brother flew in from Chicago. His [Dad's] immune system was incredibly low and my sister was a bit sick. We had a father's day dinner when he was released from the hospital that weekend, and my sister threw a fit about being asked not to come. So, my dad finally let in and allowed her to come. I started realizing that sister was not seeing the same thing that me and my brother were seeing.
In early July, he got sick and was admitted to the hospital again for the last time. By the time I got there, he was barely conscious and it just *felt* like it was time. My brother flew again from Chicago. During this time my sister and Stepmom were unnaturally upbeat and happy, as if they were completely dissociated from the world. It bothered me as I could see that they didn't realize what was happening. The fact that I had 8 days to sit by his bed and talk to him, experience this natural phenomenon with him, and face what was happening was very therapeutic to me. I know my brother was in a similar space.
One night, about 3 days before he died, I told [sister] that it was getting close. I figured she had realized what was going on at that point, but she didn't. She looked at me like I was somehow making it happen by thinking of it and seemed angry with me for saying that. Up until the moment he died, I don't think my sister or Step mom ever came to terms with the idea that it was really happening this time around.
To be fair, my sister was 24 being 10 years my junior, and perhaps her lack of experience with death and severe illness hadn't properly prepared her. However, my step mom, who had lost a child when I was 16 [our step brother], seemed completely stunned by this new development.
It was hard to see them lying to themselves about something that I thought should have been self-evident and would be so much a more enriching experience had they faced the truth. How arrogant of me to assume that their path during that time would have been made easier if they had accepted what was happening, but it just seems to me that it might have put a bit of perspective on it.
So, that's the nuance I think between the ask (when to TELL the truth) and the answer (the timing of realization of the truth).