I don't have a huge background of experience in collabs, but I've done some via Piratepad or similar sites. Essentially, the posts were written in a single sitting in one go. Two at most. Anything more, I think, would start to drag and boil down either to people not having enough time, or the post becoming extremely long. For the most part, I think if people can find a good time to get down to it should be enough to keep a reasonable pace. Becomes an issue if no middle ground can be found though.