How do you calculate the life span of patents?

Calculation of the life span of patents starts from the date on which you ___ the patent application.

The answer is Filed

A patent term is the maximum amount of time during which a patent can be maintained in force. Normally, it is expressed in a number of years either beginning from the date of grant or the filing of the patent. Example is a US utility patent which is usually granted for twenty years from the date the patent application is filed or granted