How many years should a person stay in India to get citizenship?

The Central Government may, on application, register as a citizen of India any person (not being an illegal migrant) if he/she is a person of Indian origin who is ordinarily resident in India for seven years before making an application for registration. 

Citizenship of India by naturalization can be acquired by a foreigner (not illegal migrant) who is ordinarily resident in India or has been in the service of a Government of India for TWELVE YEARS (throughout the period of twelve months immediately preceding the date of application and for ELEVEN YEARS in the aggregate in the FOURTEEN YEARS preceding the twelve months) and other qualifications as specified in the Third Schedule to the Citizenship Act, 1955. 

Further Reading:

• Citizenship in India
• Citizenship Amendment Act 2019 (CAA)