My mom will visit me in USA on March7, 2011. How long she will be USA and will be counted as my dependent reduction?

Are you asking how long she'll be in the US? I have no idea.

Are you asking if she will be your dependent for IRS purposes? How long will she be here?

Here is IRS law - Publication 501 (2010), Exemptions, Standard Deduction, and Filing Information (http://www.irs.gov/publications/p501/ar02.html)