Search Engines do not rank websites overnight from link building. It does take a little time to build quality links and search engines do take a little time to pick up or index new links. It does take a few weeks before you could observe improvement in keyword rankings. The nature of the keywords, competitiveness of the industry, age of the target web site, the aggressiveness of the link building campaign and the types of links one builds are the primary factors that determine the velocity of ranking improvements.