I should think that one backlink would be sufficient. However, if the page that links to the new site doesn't get crawled very often, then the new site will be slow in getting indexed. All I do to get my new sites indexed is submit the URLs to Google, and most of the time, it's before I've even gotten any inbound links. After submitting the URL, it has taken anywhere from about 24 hours to a week for the sites to get indexed.
Originally Posted by bogart
MeetHere, another thing you could try for quick indexing is to post an ad on craigslist with a link to your site. (Make sure the ad is relevant and in the proper section. Otherwise, it will get flagged.)