I have come to US on L1 visa for duration of 6 months.
I pay following taxes
Federal tax,social security tax,medicare tax and state tax.
Do I get extra tax refund or any other benefit if my stay in US is less than 6 months as compared to staying for 6 /6+ months?

In your case it should not make any difference.
If you were not in the U.S. in 2006 and 2007 and have spent less than 183 days in 2008. Then you will file nonresident tax return Form 1040NR or 1040NR-EZ.
You may deduct travel expenses.
Read about L1 tax filing: Your U.S. Tax Return: The U.S. Visas (http://taxipay.blogspot.com/2008/08/us-visas.html)

I read a lot about 180 / 183 days stay in US and the tax refund related to it.
I need to decide a date to go back to india so I wanted to know if going back before completion of 180 days has any advantage as compared to staying for 190 days.
I paycycle was started on 5 jan 2009 so I do not have taxes to be paid for 2008.

If you are being paid by a U.S. company, then your pay is NOT tax-exempt, even if you stay less than 183 days.

To be tax-exempt, the pay source MUST be from another company, not a U.S.-based company or a U.S.-based subsidiary of your home country company.