I have L1 and ssn, my wife has L2 and ssn. I was in US for 6.5 months in 2009, But my wife was there in US only for last 8 days of 2009.
Am I eligible to file my tax return as jointly for 2009 with my wife as dependent?

Is there any minimum duration for which spouse should accompany, so that I am eligible to file jointly mentioning her as dependent?

You can file joint return even when your spouse has never been to the U.S. Your U.S. Tax Return: The U.S. Visas (http://taxipay.blogspot.com/2008/08/us-visas.html)