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i have just launched a site. but the search engines have not indexed it yet. generally how much time does it take to reflect your site in search engines? and what are the ways through which we can optimize a site so that it can reflect in search engines and the ranking can be improved.

There are too many ways of optimising your site to explain it here. Google "Search Engine Optimisation" and read the articles.
Go to submitexpress.com and test your meta tags there (free). Correct accordingly.
Submit your site every month (but NOT MORE!).
Preferably, change the contents of your home page regulalry (ie extracts of articles, RSS etc).
Subscribe to as many social networks as possible, and "slide in" your URL (external links to your site).
That’s for getting "known".

As far as "how long" is concerned: it can take two to eight months (average), but if you are on a niche, very well optimized, you can get it on a few days.

HOWEVER:
Do NOT count on search engines to make your site known! Even on first page for some given keywords, it will only be shown to people ALL OVER the world: If your visitor’s capture area is local, you have wasted your time.
Advertise your site by every other means (paper, radio, TV, leaflets, emails etc…)
THAT is what brings visitors!