H1 B time period to start green card

[mesudhir]

My first H1 B stamped for Mar 2008 - sep 2010 but (I 797 is for Sep 2007 - Sep 2010).

With this H1B I was in usa bet may 2008 - Sep 2009.(1.5 months)

After that I was there in India till Jan 2011.

Mean while my next H1B (new) started from April 2010 (I 797) and got stamped for Aug 2010 - Apr 2013.

My question is:

Now is my total 6 yrs H1B time is over by April 2013 or will I get any extension after Aril 2013.

If not do I need to start immediately Green card processing.

Usually How the H1 term is calculate? Please give some information.

Thanks in advance..