The simple answer for the time it takes for a background verification process is "as long as it takes." Some are easy to do, others more complex, particularly if the candidates have not given all the required information or have lied about previous employment, education, etc. I speak from experience here.
You need to bear in mind that it is not always your prospective employer dragging their feet; they may be waiting on answers from other people who are not in urgent mode like you. I have often read in articles that almost nothing is "urgent" in India. It gets done when it gets done, no sooner, no later.
So park your anxiety, and get on with your life until you hear back from them.