As for me, I usually have an idea but I also tend to follow the story. once I have up to 20 or 25 pages with 3-4 panels on each page, that usually seem to be enough however, if I feel like there needs to be more explanation, then I'll add one or two pages depending on what I'm trying to explain.
The goal here is to make sure you have visually explain what you need to explain and if the chapter drags to 30 pages, that's also fine as long as it all makes sensens and ties into the story.