The dev has said there are about 15 chapters planned, and the last one released was #9. It might go past 15, but also might not. So there is a rough map but no completion date. Updates are slow, several months to about a year depending, but the dev is good at communicating progress to supporters.
The dev is professional artist making MPP as a side project. IIRC he's doing everything on his own (coding, images, etc.) including 100% hand-drawn art, with a handful of specific items contracted out. He could produce updates faster if he had a team and/or focused solely on updates. But I know some artists personally and I suspect not having 100% control (including when to contract out items) and/or not having other stuff to work on would drive him batty, ultimately resulting in an abandoned project.