Besides when I started mirroring from Smackjeeves, I've never really had a buffer ever. I worked on a page a week and that's it. If I was lucky or weirdly productive I might get 1 or 2 pages ahead, but I wouldn't call it normal by any means. I also don't write my comics, I work with a writer. The writer sends me full scripts for each chapter, so I'm not so worried about getting worked into a corner story wise.
I know Terri, the writer for my most popular series Goblins of Razard, has most of the script to the end of the story drafted. The writer for the other series Lockhart I work on is sort of similar, sending me a whole chapter at a time. For him I try to sketch out all the pages in thumbnail so he can visualize the story and make changes before I start on actual pages. So in some respects I'm ahead by quite a bit, but for actual totally finished pages, I hardly ever have those ahead of schedule.