Term of Patent
The standard term of a patent is 20 years from the date the patent application was filed. All rights of the patent holder end when the term is over. Unless the term has been extended, the invention falls into the public domain after 20 years, and anyone can use it.
The situation is a little different if the patent has expired before the end of the patent term because the owner failed to pay maintenance fees. A patent owner can petition to revive a patent that has expired for failure to pay fees. Using someone else's patented invention whose maintenance fees weren't paid is allowed, as long as the patent has not been revived by its owner. But it is wise to frequently search the United States Patent & Trademark Office's database to check the status of an expired patent.
Just because a patent has expired is no reason to assume that you are free to use the invention. The patent owner might have filed another very similar patent, or might have filed for a continuation and let the original patent expire. Under these circumstances, you might be liable for infringement, if you are making or using a protected invention. Conducting a patent search for similar inventions can prevent liability.