Thanks to everyone for valuable inputs.

marconi:
I am on 20hr CPT (Part time) and I guess there is no restiction on how long you can work on part time CPT.

garibbakochhora: 
 I had doubts after I applied, so I asked the manager to drop my application (after interview). But by that time they had already selected me. So this guy (manager) went up to HR, ISA officer and my professor and talked about the issue. Finally it was ISA officer who suggest that I can work by combining 20hrs + 20hrs and everyone agreed to it. Here we need to re-issue CPT every semester. I already did twice and ISA officer was happy to know that I am doing good over there.
I even asked ISA officer that will it be problem at port of entry and he said "just tell them, I had to work more than 20hrs on campus so part time cpt was issued". Now the time to go home is getting closer and it is really making me worry.

Guys do you have any idea Where can I get assistance from a lawyer who is good at all these stuffs.

Have a good long weekend !!!