I'm proud to say, with the implementation of the "Downhill" level and a "Cpt. Greenbeard" boss fight, the first quarter of the map/campaign is done. There's an into level, store, 5 regular levels, 1 boss fight and "the underground", so the game is finally shaping into something solid. The plan was never to fill all four quarters at release but only three - some 25 to 30 levels total - the last peace of the map was always meant for additional content some time later, like bonus missions, seasonal content and so on, maybe even an unrelated mini-story. To be decided later.
So, at minimum, I have to make another 19 levels and 5 secret levels to finish the game, not counting further boss fights. In theory, if my day job doesn't get me too busy, I could finish by the next summer... or autumn. Otherwise, hard to say.