I know what you're trying to get at with your question... I'm not trying to be a smartass, but the best answer is, "it depends."

Think back to your high school / college paper writing days and try to remember how long it took you to write essays versus your friends and classmates. People can have the same information and analytical ability, but they take vastly different amounts of time to write up their analysis. Some people are the type to read and re-read what they've written, obsess over how they say things, etc... while others just plop out what's in their brain and it comes out "good enough."

On top of the writing, a blog can be so much more depending on what kind of experience you want to build for your readers. You can include pictures, video, podcasts in your article posts - whether you find these or make them yourself will take time. Depending on what blogging platform you use, you'll be able to add plug-ins or gadgets to increase functionality or interactivity. You'll also need to moderate comments and respond to comments.

I'll go ahead and throw out a really rough estimate, (but you have to keep in mind it's based on my writing ability / speed) and say that it takes about 1 to 2 hours to crank out 400 - 600 words. To be successful, you probably want to update at least once a week if you're producing video / podcasts and probably 3 times a week if you're just writing.... so you're looking at about 3 - 6 hours a week + any behind the scenes management and research that you'll be doing. Less than a part-time job... but more rewarding? That's a whole other question... or potentially a blog post.

I tell people that starting a blog is a lot like giving the keys to a racecar to you. The question isn't how little you can do to achieve your results, the question is how quickly do you want to reach your goals. A blog that posts 10 times a week has ten times more potential than a blog with one post a week. As well, utilizing analytics, search engine optimization, promotion of your blog, and participating in the blogosphere can all accelerate your blog's authority.

The question isn't how much time to spend... it's how much time are you willing to spend and whether or not you're using that time wisely!

Its up to you! Some people want to write once a week, some once a day, some on a regular basis and some sporadically. It also depends on whether you want to actively seek other readers for your blog, or whether you'll just share it with friends and family members; whether you want to use sponsored posts, put Adsense on your blogs, join in on memes, etc. As for developing an article, it depends on your writing talent and experience! I would say, any time you decide to sit down and write an article, it may take about half an hour to an hour.