I actually spent about a year rough-draft-storyboarding every page of my comic before I let myself start drawing any final pages. But this is because I'm the sort of person who gets stressed out by uncertainty very easily; I knew that if I started trying to keep to a weekly schedule without having an exact path, I'd get discouraged the moment I ran into a plot point I wasn't sure how to address, or start going off on a tangent, decide I hated it, and stop drawing the comic entirely.
Forcing myself to outline every scene and dialogue before I started gave me some freedom - I could put a draft down for a couple days and come back to it when inspiration hit, without having to worry about the next update being due soon. My case is probably an extreme version though, and I have a relatively short comic I was actually able to plot from start to end. Everyone's situation varies.