If you feel super confident with the prep material you have now, It's about time you started working on it! 
But, for one like myself. I'm currently trying to build an audience with my first comic that I'm currently posting. I'm also using the first comic as my practice-comic because let's face it, you make the most mistakes in your first work. For example, I had trouble with fonts at first, think about it... FONTS! At least I won't make that mistake anymore.
At the same time I'm also trying to finish up writing my other comic, which has an overarching plot line with multiple main characters. Obviously I rather not start posting the one I'm writing on because I see its potential and would rather polish up the prep work to the point where the characters, story the world feels real. I've drafted 2 of 3 acts so I have yet to reach the revision phase.
In the end it all comes down to an author's confidence or purpose with their comic. If they're doing it for harmless fun they'll improvise and post whenever they want to. If they have story that they want to execute well, then it's best to a have prep material to avoid plot holes, lack of world building, character inconsistencies and retcons.