An interesting semi-spoiler comment from Mary in the official companion:
"I didn't want to sign on for a death scene in the first episode after the miniseries" she explains.
"So I talked to them about it, and I think we all agreed that the character's journey would take a few years to explore, at least. I certainly felt that she was going to be an intrinsic aspect of the story. But beyond that, they couldn't really say how the journey would evolve or exactly how long it would take".
As I said semi-spoiler