How long does a patent last?
Patents last 20 years from the date of filing of the first patent application that includes the claimed invention. Patent lifespans used to be 17 years from the date of issuance, but that term was changed by the General Agreement on Tariffs and Trade (GATT) agreement in 1995. Older applications are subject to a grandfather clause that gives them the later of two possible expiration dates.
How did they come up with 17 years?
Back in the days of yore, people apprenticed themselves to master tradesmen to learn a trade. The apprenticeship lasted for seven years. Out of custom and courtesy, the apprentice would not practice any special little tricks or techniques, those secrets that his master had taught him that weren’t generally known in the trade (i.e. inventions), for a period of two apprenticeships, or fourteen years, after the apprenticeship ended. The abstinence period assumed that the master had developed his inventions smack dab in the middle of the seven year apprenticeship, or three-and-a-half-years before it ended. The total length of time that an ex-apprentice would refrain from using his master’s inventions was therefore (3 ? + 7 + 7) years, or 17 ? years. Rounding down gave a 17 year monopoly for the inventor.