I'm three weeks into publishing my comic, currently only at a rate of two pages a week so I've got six pages out so far having started on 1st October.
The story came to mind just after Halloween of 2018, I wrote character profiles for main and minor characters over the course of a month and doodled pictures of them.
Over the next month or two (ish) I wrote down the general plot from beginning to end, I assured myself that I was not going to draw anything until I knew exactly how the story would end.
Sure, I can change things later, but if I don't know where I'm going to begin with I might fall into the trap of making shit up as I go along and that might become difficult. It also makes it far easier to avoid continuity issues and easier to have fun little bits of foreshadowing that readers can enjoy when they go back to look at old pages.
It was January by the time I had a full story worked out. After that I then took the first two sections of it and turned them into a script. For me that was basically novel like to read, describing what could be seen and all the dialogue. This part took a month or two itself. (I want to stress that my timescales are restrained by working full-time and having a family life too!)
So in total I took about 4 months just writing. It's taken another 7 months to feel confident enough with my drawing skills to actually start publishing. The result is below (because who can avoid a self-plug!) =D