general time in a year during which to give GMAT ?

Strategically, you should take the test scheduled around when your personal application deadlines are set. For most schools, you can take the test right up until the day before the app deadline. I suggest that you plan to take the test twice, which means you'll want to take the test for the first time about 40 days before your app deadline at the latest. (The GMAC requires a 31 day waiting period between takings of the GMAT.)

Now, let's talk demand for tests globally.

GMAT demand is pretty flat, meaning there aren't large peaks and valleys in test taking like there are in other standardized tests such as the SAT. That said, most people take the test right before their apps are due, and most apps (for high scoring students) come due in the first quarter of the year, so there is a rush in Dec, Jan, Feb among high-scoring students.

Thanks for the brief.
What I am looking for is to appear for 2013-2015 Class of MBA.
I have seen R1,R2 for some Schools.
What are these deadlines and What do they mean ?

Deadlines for each round vary by school -- I'd recommend checking out the websites of the schools you are interested to get a sense of the actual dates. They typically don't change too much (or a tall)from year to year.

Also note that while a lot of schools do have application rounds (ie R1,R2,etc), they actually do admissions on a rolling basis, so it behooves you to submit applications as early as possible in most cases.

To echo what Tom said, I would be sure to take the GMATs well in advance of your application period (perhaps 3 month or more before you plan to start applications) so you are not overwhelmed with juggling your job, application materials and GMAT stuff.