Patents are valid for 20 years from the filing date of the first non-provisional patent application to which priority is claimed. After the patent issues, the patent owner must pay a patent maintenance fee in a window between 3 years and 3.5 years after the issue date of the patent, between 7 years and 7.5 years after the issue date of the patent, and between 11 an 11.5 years after the issue date of the patent. The patent maintenance fee can be paid for up to six months after each window with a payment of a surcharge.
The purpose of the patent maintenance fee is to weed out patents that are no longer in use. The patent maintenance fee increases for each window, with approximately a doubling in patent maintenance fee between each successive patent maintenance fee.
A patent attorney can assist you in tracking these fees and may refer you to a patent maintenance fee tracking company to track fees for you.