Serious answer, @vantran: a LOT.

I can't give you the exact number, because I've written a rough draft a few weeks ago, and then tweaked it a couple of times. And yesterday, before posting it, I still spent like 6 hours on it.

I would say I spent at least a full day on it, like 10 hours minimum. But...keep in mind that:

(1) I'm not a native English speaker: even though I'm fluent in English, it takes more time for me to write in English.
(2) I'm a perfectionist: and for this post, I really wanted to give it my very best shot.

I checked your profile, and noticed you are still at the very start. Welcome!

If I were you, I would hold off on posting your IntroduceYourself post for now. Instead, do this:

• spend a few weeks to really get to know Steemit and its community. Read posts, interact. Leave meaningful comments, ones that authors notice and appreciate.
• start writing posts. Don't focus on making money. Consider this Kindergarten. Learn how to edit properly, how to use Markdown for editing, how to write well, how to use images.
• when you leave a comment, don't ask for upvotes or follows. Also don't upvote your own comments. It's lame, and your upvotes aren't worth anything anyway. But what it will do is damage your reputation.
• follow established Steemians, that write posts that perform well. Ask yourself: how are they creating their content? What can I learn from that? You'll soon notice that most posts that perform well are written by Steemians that have built their following for a year or so, by publishing quality content consistently. Keep this in mind when your first posts don't make any money.
• Join the Minnow Support Project.

I did all of this before I wrote my 'IntroduceYourself' post.

Hope this helps!