It depends on the story really, like for my sit-com-esque series I only planned probably 40% of the way in before starting to draw it. That's because it's a touch more episodic with a meatier plot that develops the later it goes. Like at its core, it's just a girl interacting with demons, but the latter half of the series deals with how they got there in the first place.
For my romance story, it started on a whim, I just really liked these two characters I created so I started posting some pages? After the first two updates I then really buckled down and came up with a general plot that I fully fleshed out after a month, and then gradually filled in dialogue and nitty gritty details after another 3 months.
For my other compact stories, I like having the whole thing written out before starting. Of course the dialogue I write isn't set in stone and could be changed as I'm drawing.