And just out of curiosity, how close are you to a release date?
I started out with a simple answer, but then went on rambling, so feel free to ignore most of this. I'm pretty much just talking to myself...
I've been wondering when someone was going to ask.

My honest answer is "I don't know". Everytime I enumerate what's left in my head, I get to the list of UI glitches (for example - deleting a dungeon level from the middle of a dungeon doesn't always reset the dropdown box that lets you quickly navigate to different levels. The result is that the dropdown list may read "|Level: 0 | Level: 1 | Level: 3 | Level: 4|" if you've just deleted Level 2. It only happens sometimes, and I have no idea why. Another is in the exvars listing. If you double click the 'cell' - think of that list as a two-column spreadsheet - it's supposed to launch a helper dialog based on what's in the cell before it; so if you're editting a 'target' exvar, it launches the Target Finder dialog. That part works fine - the glitch is when it comes back. If, when you double-clicked, you entered the 'text-edit mode' of the cell - that is, the little blinking caret has appeared and is waiting for you to type - the results of the dialog don't show up; they're "hidden" underneath the space provided for to type. It's there, and it'll appear as soon as you click anywhere else, but it's kind of dumbfounding the first time it happens and you don't realize all that work you potentially put into the dialog isn't gone. I've tried all sorts of ways to fix it, and ultimately it comes down to a problem with .NET itself - which, for some reason, completely refuses to acknowledge that a text caret exists).
There's also the list of things that I need to figure out how to inform the editor about - this is all direct result of it not being able to understand DSB/Lua (another example: for making champions, you need to select a portrait. How should this be done? If I force designers to type them out, I'm looking at a good source for hiccups in the generated code - bad spelling. I'm currently using a dropdown list, but how does it generate that list? Yet-another-config file? The same thing for level-wide wallsets and classes in the 'opby_class' helper (which currently doesn't exist - no helpers exist for 'opby's yet, which is probably a bit more than a minor UI glitch).
The only thing I consider major left to do is custom wallsets - those that are placed in the dungeon on a tile-by-tile basis, not level wide. That really should be done now, because to add it later is going to mean changing the data structure of the dungeon, and that's probably going to mean that old saved dungeons won't work with any updates that include it. I guess the Opby helper is somewhat major, too, but it's not a change to the dungeon data itself, just in the way designers interact with it.
Now, you may be wondering why I'm so fixated on these dumb little exvar helper dialogs (and I'll admit it, I am) - it's because there's really nothing else for a new dungeon designer to learn from. If you wanted to learn how to make a dungeon in CSBuild, the most common piece of advice given is "look at how the DM dungeon is built" - that's how you did it. The first time I used CSBuild, it took me a long while to figure out how to make a sconce with a torch in it. The only reason I ever learned was because I had the DM dungeon to learn from. But this editor has no pre-made dungeons to learn from - and because it doesn't understand Lua, it can't use, say, the ones Joramund has made to help designers along. So, the choices for a new designer are to learn all about how DSB/Lua builds dungeons before ever touching the editor, or giving them lots of help along the way.
The trick is to do it in such a way that people who've already passed the learning curve aren't slowed down by all the dialogs and helpers and such. For example, when you use RTC's editor to make a dungeon, to put an object inside the dungeon you have to pass through a series of dialog screens. The pop-up menu can activate the tile editor, from there you can pop open another dialog for objects, and from there, more dialogs for more options. That's fine, there's nothing fundamentally wrong with how it works. The only problem is you have to do it
everytime you want to add something. There's a strict order-of-implementation that must be followed. It certainly makes RTC's editor easy to use (it's impossible to mess up adding an apple to a dungeon), and it doesn't take away from any of it's flexiblity, but it implies that flexibility even if you don't want it. You can't just say "Hey, put some cheese in the corner, there" - you have to say "Okay, I'd like to edit this tile; I'd like to put an item on it; I think it should be cheese; Yes, just cheese". Now, because you have all those steps, it's quite possible to make "super-cheese" - by editting the number of charges or whatever when that particular dialog appears (and it does, everytime, because it didn't, you'd have to go back into the dialog cascade and find it, which would be tedious). But usually, you probably just wanted cheese.
Now, the editor only implements the flexibility of DSB on one level - exvars - but I still wanted it to be as unobrusive as possible. This is the reason for the current "flat" design (its also why the UI was changed - 'flattening' the interface ended up taking up alot of screen space - hence the ability to hide the side-panels as tabs along the sides, and have multiple dungeon level maps open at the same time). Almost everything you need to make a dungeon is present and visible from the moment you go to the File menu and select "New Dungeon...". You can add levels, draw your floor plan, drop in instances, edit their exvars, make containers, whatever, without ever looking at another screen or openning another window (there's two exceptions to that - creating champions requires openning up the roster, and making containers inside containers - like a skeleton carrying a chest with a flask inside - means openning up the Container dialog). I wouldn't recommend it, because common exvars like 'target' can get tedious to type out again and again, you have to remember to give instances unique Id's and blah-blah-blah -but it's certainly possible. There's also not a single case of one dialog creating another one. Because it's all flat, each one is seperate, so you don't have to follow through cascades you might not know how to navigate ("Say, where is that screen where I edit charges, anyway?") to perform tasks.
Alot of this is, of course, a result of Sophia's work with and the general "flatness" of DSB/Lua itself. Exvars give you lots of options, without really ever crowding each other (certain sets, such as 'opby', 'target', and 'msg' are used together and a pretty worthless alone, but that's really on a case-by-case basis), and without requiring lots of things to be setup before hand. Pretty much, the only thing you need to setup an exvar is an instance to set it up on - the editor tries to reflect that simplicity as much as possible.
Anyway, I've veered way off the question, and you're probably all bored to tears, so to answer you: Maybe a week.
*[I'm going to note that I'm implying nothing bad about RTC or CSBuild or any other editor or clone or programmer out there. They've all put in a huge amount of effort. If I make minor complaints, I make them with massive amounts of salt, and completely aware that if they hadn't clocked in those hours beforehand, I wouldn't be here to complain.]