i like to script as far forward as i possibly can; it means you know where youre going, which is important, and gives you motivation, because even if youre bored with the bit youre drawing now, when its done youre closer to drawing a really awesome bit you have planned.
im currently scripted six weeks ahead of the present and have vague plans for up to 13 weeks away. i want to write out the entire arc, so if i get caught up in gcses or die suddenly, someone else can pick it up. which means scripting it first off like a story, and then doing heavily detailed panel scripts, and leaving sketches everywhere.
i dont do storyboards, however i used to. sometimes, if im stuck, i sketch out the panels to see how id fit it in, but i find that if i do storyboards i end up wanting to trace this bit and that bit because i like them so much, and it doesnt turn out as well. waste of time, personally.