Let's see. Back in 2011 we took about 9 months to write the entire first draft. Most of 2012 was spent editing the writing. I also did a few "test" comics at this time to try and figure out the style I wanted to use for Midwinter. Did most of the major concept art at the end of that year, too. We started actually producing pages in spring of 2013 and have produced a chapter (40 pgs) every year since!
Only now, 4 years later, am I thinking I could reasonably try to do 2 chapters in a year. I'm working at home now and getting paid a lot more for the work I do, so I'm hopeful we'll be able to do better!
We're in a state of constant editing though. Things change literally up until it gets put on the paper, and we've rewritten every scene so far (sometimes more than once).