When it stops implying other people. Role-playing is a social activity, that is, involves 2 or more people. For this to happen there must be some degree of vagueness where both may contribute to it. Therefore, the main idea or pitch the first person had will invariably mutate with the input from seconds. The one person that had the RP idea does no longer have full editorial control over where the roleplay goes. 

I am reminded of a poster in the forum of the infamous hacker known as 4chan. This gentleman had a very elaborate map he liked to show around, and also spoke lengthily of his plans about his setting. It all begged the question: when do the players come in? How do you turn this into a role-playing game? The poster then admitted that it was never the plan; this was to be a book. So I suppose that until the idea becomes too elaborate for other people to have casual input. It stops being a role-playing idea when there's no common language. A wiser man than I wrote about this. I'll link him here once I can. Edit: https://monstersandmanuals.blogspot.com/2021/05/of-krynn-and-urth-shared-vs-singular.html?m=1

Of course, in truth a novel, book or comic will have more than one person working on it on an official capacity. I imagine here the difference is a certain asymmetry between them, while a group of roleplayers is much more symmetrical.