All Subject GRE tests (e.g. the Physics GRE) are paper tests (offline). There are 100 multiple choice questions and you have 170 minutes.
For more information, you should first go to the official ETS website (ETS is the company that makes and runs the test): https://www.ets.org/gre/subject/about/content/physics
The test happens only 3 times a year (September, October, April), but not every location offers a test on every date. This PDF (https://www.ets.org/s/gre/pdf/gre_pbt_center_lists.pdf
) shows the test dates available from 2015-2016. Make sure you scroll past the General GRE test locations to the Subject GRE test locations. For the 2015-2016 testing year, the subject GRE was only offered in India on the October test date. If this is the same for the 2016-2017 test year, then the next test date available in India will be October 2016.