When does it end?
After all that hard work, the applicant has been granted a patent. They plan to pay the maintenance fees on time. So how long does an applicant get to keep the rights to exclude others from the subject matter in the patent? It seems like it should be forever with all the hard work that went into gaining the patent. But it's hardly true.

The term for plant or utility patents starts on the date the patent issues and ends 20 years from the date the application for the patent was filed in the U.S.

Although that statement sounded simple, it's not, you'll want to pay attention here.

The 20 year countdown actually begins on the application's filing date, not on the date the patent issued. That means if it took three years for the patent to be examined (which is very realistic), you would only have 17 years from the date the patent issues left on the term. The point is, the PTO starts counting immediately after the application is filed. After all, the subject matter will be protected even during the patent pending period.

That's why it's very important for an inventor to start marketing their invention immediately after the patent application was filed. 

If a plant or utility application contains a reference to an earlier filed application, the patent will expire 20 years from the filing date of the earliest filed application. That means a continuing or divisional application will expire 20 years from the filing date of the parent application which could significantly cut into the patent term.

Design patents run off a different schedule. Design patents expire 15 years from the date the patent is granted. So, for these, it doesn't matter when the application was filed, you only need to consider the date the patent was granted.

Patent Term Adjustments
Patent terms may be adjusted under certain conditions. One circumstance that may result in an adjustment to the patent term is when the PTO takes too long during the prosecution. The PTO has now implemented a 3-year policy. If it takes them longer than 3-years to examine the application, the patent term will be extended. For every day over the 3 years that it takes to examine the patent, the term is extended by one day. Note that this rule only applies to utility and plant applications. In addition, it only applies to the original application, not to continued applications.

The patent term is also typically adjusted if the patent is delayed by a derivation proceeding, an appeal, or a secrecy order. In addition, the patent term may be adjusted due to time spent waiting to gain government regulatory agency approval (like when the FDA has to give approval for a drug or therapeutic agent).