Once a patent is issued it has a 17 years life span. The law tacks on another 3 years for issuance of the patent by USPTO, so there is a provision that a patent can have up to 20 year life span from date the "Application" is filed.

It can be "Extended" at any time before its time expires. This is done by applying fo an "Extention"...

Of course it make no sense to apply for an "Extention" in the early stage of its life span. On the other hand if you consider the fact that under the law the USPTO can take up to three (3) years to respons, it makes sense that applications for an "Extention" should be filed no later than the 13th year of its existence...

Gil...