A development edit – or structural edit – is a crucial step to make sure the content is as great as it can be. In this stage, the author will make substantial changes thanks to the insight of an experienced and skilled editor. It may take up to four months, sometimes longer.
The first step of the publishing process is arguably the most important one because it lays the foundation for everything else. After submitting a finished manuscript, the author works closely with a development editor. This kind of edit makes sure the manuscript is the best it can be. It deals with the content and the narrative style: the plot, the characters, the structure, the voice and so forth.
The scale of the development edit varies. Often, fiction titles have more in-depth edits than non-fiction because of the complexity of the plot, characterisation, structure and continuity.
For non-fiction, development edits focus on structure and narrative as well, with a particular focus on clarity of argument. They may involve fact-checking and rhetorical aspects, too – especially if it is an opinion piece.
As you can imagine, no one involved will want to rush this process, but patience at this stage will be rewarded. This takes time; how long exactly is hard to predict – usually around four months but sometimes longer.
The next step is the copy edit.
(And here is some info on the publishing process as a whole.)