Hello Amkohl,

Since it is the first time you are writing a GMAT, you may have missed on timing techniques and the strategy rules to drive your Quant and GMAT. But these factors can be practiced and perfected in time.



So here are my answers to your questions :

1. Time for preparation
I am also a working professional so i know it is difficult to focus on GMAT everyday and for long time. So i would suggest studying material over weekdays and practicing content over the weekends.
Ideally, you should devote 2 hours per day on studying concepts. Take a set of problems on weekends and review your performance on these sets over the weekend.

2. Reference books
Official GMAT Review by GMAC - Can be brought on Amazon or MBA.com
(if you are indian, i would suggest Flipkart.com / Junglee.com as the best place to buy these books)
If you can afford, Manhattan Strategy Guides - Set of 8

3. Joining any classroom course
If you are confident that you can pull off a good score without any help, go for it. All GMAT courses are quite costly (Both Online Courses as well as Classroom Courses). If you think you really need professional help, i would suggest Manhattan GMAT / Veritas Prep - Both are highly rated on Beat The GMAT forum.

4. Study plan
As stated above
Weekdays (Mon-Fri) : GMAT Concepts (both Quant & Verbal) for 2 hours
Weekend (Sat-Sun) : Practice Sets and Review Errors - Sit down for like 4 or 5 Hours

Regards
Penguinfoot