Every patent in India lasts for 20 years from the date the applicant files a patent application with the Indian Patent Office, irrespective of the type of application. Even if the application filed is a Provisional Patent Specification, the Indian Patent Law considers the date provisional application as the beginning of the term of the patent.
Once the patent has completed the period of 20 years, the claim of rights over the invention cease to exist. Hence, the invention is open for public use.
While the period for all patent application in India is 20 years, the date from which the patent terms begins can be different depending on the type of application.
Convention applications are granted for a term of 20 years from the date of filling in a convention country. The deciding factor is the date of filing of the first priority application. A conventional patent application can be filed in India within 12 months of filing the priority application.
Suppose, the first priority application filed by you in the US on May 2018 and the conventional application was filed on August 2018. Then the term of the patent will be calculated from May 2018.
Expiry for PCT application is 20 years from the International Filling Date.
Patent of Addition: The term of the Patent of Addition is based on the main patent application for which it has been filed.
Divisional Patent Application: Expiry for ordinary divisional application is 20 years from the date of filing for patent application.
The term of a patent granted before 2003 differed from what is granted today. Process patent received protection for 5 years from the date patent was granted and product patent recieved protection for 14 years.