Few hours; write a few articles, submit bookmarks, tweet it, ping it, make and submit rss feeds, ping those rss feeds. The hard part isn't indexing; it's rankings
Sometimes I find my posts here on DP within hours on Google.. maybe even quicker. I've had my website added within 48 hours.
@schmedes2 you are absolutely right, getting index doesn't matter whereas getting SERP is something what matters. For a fresh domain, it usually takes 24-48 hrs whereas when I update a blog post and ping it, it get index within 10 minutes.