Heya, I think it can vary so much, how big an artist chooses to make their buffer. It can depend on so much.
For me personally, On inkblazers, I would release 4 new pages per week for the current story (its at chapter 9) and then in pencilled manuscripts I'm part way in to Chapter 13. So this gives me around four months worth of buffer.
For you, you'll need to work out the size your buffer should be based on how much you plan to upload. Will it be a chapter at a time, or a few pages a week, or maybe just one page a week? If your going to release weekly, make you sure you have as big a buffer as humanely possible! You never know when life will get in the way, and you also need to remember that you can't get complacent, and must maintain your buffer by working ahead everyday, as well as completing the current work.
I know its a lot of effort, but a buffer will save you a lot of stress and pressure, and your doing the right thing by asking around, its a good topic. I wish you luck with your comic.
Hope this helps in some way.