I cant really give a timeframe of how long it takes me to write for my chapters...I usually write until I get to a point where I'm stuck and I need to let my mind freshen up. I'll step away for a period of time- then come back and finish the chapter off. On chapter 2 of my webcomic, it took a whole 4 months before I was able to come back and finish scripting. I was working on the half that I had already finished; I took a month break a few pages shy of my previous stopping point coz I needed to rest my arms- and finished writing the chapter while on that break.
On my current chapter, I took a month to work out some "in-between" stuff with the script- I had the beginning, but I was trying to hold off re-introducing the villain until the 4th chapter; I decided to bring back the villain in the current chapter, but split up the side story, so that I could keep readers locked in until the end(the actual villain fight will take place in Ch 4).
So far my chapters have been averaging about 40 pages- I do a page a week, so it takes about 7-8 months per chapter; longer if I take good number of breaks(which I have needed to because of sore arms/wrists/hands).