Change of employer, H1 extension and what-if-laid-off related question

[coolwave1447]

I am in US on H1 visa. I have completed almost 5 yrs in total on H1 in US.

At present I work as a contractor. But, now, I have an offer for a full time employment and am stuck in a dilemma.

In case you know the answers to below 3 questions, it would be really helpful if you could take out time and reply:

1) I had H1 visa valid till Nov,2013. In May,2012, I changed client and applied for H1 ammendment, I got only 1 yr client letter. So, when my H1 ammendment got approved, I got new I797 and I94 valid only till May,13. Now when this end date nears will I need to apply for H1 extension or renewal of visa? will I get a new visa for 3yrs or only till Nov,13?

2) I have my I-140 (2nd step of GC process) approved with current employer. Say, in Jan,13, If I change my employer and new employer files for my H1, will I get H1 for 3yrs or only till Nov,13?

3) If I change my employer, join a new employer who does my H1, then, say, lays me off or company closes in 6-8months, then how much time do I get to find a new employer and get my H1 done before I no longer am allowed to stay in the country?