How many screens are you aiming at?
I don't really have a fixed number of screens I'm aiming at. I know what the world of Falling Water *looks like* (roughly), so I'm aiming to build the world as it exists in my head. I already know, for example, that I have built (nearly) all the open surface area that exists in it now, at least for parts one and two; there's just a tiny bit of sea and islands missing and then there'll be only caves left for me to build. (I already miss the open air... and the vegetation. Though there's some vegetation in the caves, too.) It's really hard to estimate how many screens I'll end up with, though, especially as there's an entire... 'continent' (which will constitute part 3, because there's a very... natural break in the story there) that I haven't even started yet.
I think parts 1 and 2 may come in at something between 500 and 600 screens, eventually (that's ca. 110 screens for part 1, and the rest is all part 2. I initially meant to split part 2 into two parts, and I may still do that, in which case the aforementioned part 3 would be promoted to part 4, of course. ;-))
The real problem with the level at the moment is the story, not completing the map. I know, more or less, what the map will look like, and could probably build it in a matter of a few months - but the story's still elusive, except for the very vaguest outline. I'm also contemplating scrapping a part of that outline, which means that the actual shape of my world has stopped making quite as much sense. But it's too late to change the shape now, even if another shape would make more sense.
Because right now things look like a beautiful ever-expanding world. As a matter of fact, it got me thinking of how nice it would be if the protagonist found out, at the end of the game, that there is no end, that the world is being created all the time, more and more and more layers, etc. Eh.
That, alas, is impossible, for reasons that are quite central to the story. However, I can already see myself starting another big project when Falling Water is finished. Building KS levels is the closest I've ever come to being able to recreate the strange worlds I see in my dreams, and there's still plenty where Falling Water came from...
I may have to learn how to make my own tilesets, eventually... (although it's amazing how much you can accomplish with the public tilesets, really!)