This is applicable if you are running your own blog. The problem is when you are just an employee and your employer is giving you a specific time to finish certain amounts of article. Fortunately, I am the boss of my blog.
Don't worry too much about speed; quality can more than make up for speed. If you write a really good article, it's much more likely to attract attention. Example: I put a lot of effort into writing this post about why GDP is a poor measure of wealth and prosperity: I spent almost a half a day on that post, and worked on it on a few separate occasions before publicizing it. It's a relatively unknown blog and I've put little effort into publicizing it...but without much publicity, it got submitted to stumbledupon and it got over 2000 views in one day. I didn't even submit it! By contrast, articles and posts that I have whipped up quickly have never gotten that kind of attention.
1 to 1.5 hours are enough for me to write 2 to 4 articles. It really depends on the topic of the post. I try to rush the main purpose of the post into readers mind. NO TIME WASTING SENTENCES.
If writing a high quality articles, it would take a lot of time for me to write in. But with my own thoughts or just daily life ramblings, I could finish as early as 10-15 minutes.
I generally write 400-800 articles in 1-1.5 hour, but based on topic sometime it takes only half an hour and sometimes more than two hours.(English is not my first language, I am from Bangladesh)
It depends on the mood and available resources about the subject. It takes 20 minutes to hour and a half, including image processing (finding and cropping to a standard size that was set in 2nd version of the site) for a 450-550 words article.