Fullness Threshold

I just read an article which said that a PhD in education sciences (presumably in the U.S.) takes 13 years on average. And, here I am trying to finish one (albeit not in education) in three years. On some days, such as today, it feels like a tall ask. But, I feel like with a project such as mine, where the end-point is not obvious, an end-date is required because otherwise the project can go on forever. An end-date is a rough mental marker for when to stop and what to leave for later projects. Of course, just because the end-point is not obvious doesn't mean I can simply stop writing when my time runs out. As I have written before, I aim for a feeling of fullness that comes with a writing job done to my satisfaction. This goes for even a blog post, so, naturally, the expectations and "fullness" threshold is much higher for a PhD thesis. And, of course, I hope I reach my fullness threshold before I reach my "fed up" threshold. For the paper I just submitted recently to a journal, the latter came before the former, but fortunately the two were quite  close to each other so I could submit without a guilty conscience.
When people ask me if my PhD is on track, I don't know what to say. If only there was a track with clear milestones. But, that wouldn't be any fun in my discipline. There is madness in the method and in laborious stretches such as where I am right now, I try to remind myself of this maxim.