Several hours if I include research and careful editing. Or even days if there are interviews involved.
If it's an opinion piece, a couple of hours at most, with a short break in the middle, will do.
Here is what I like to do and it saves me some time.
I will hire someone to write posts for me and do all of the research....I then rewrite it using their research. Some people say that I am duplicating work but I don't see it that way. I am paying someone to find references for me and then I make the content great. As far as time, it doesn't matter much to me. My content will bring me value for years. How its done on the backend is irrelevant as long as the final product is top quality, relevant, valuable content to those who will consume it.
I can tell you that if you have a site that's sole purpose is ad revenue, you will have some tough times over the next few years.