When should a patent application be filed?
Since the United States is now a first to file country, you should file your patent application as soon as possible. You must file within one year of the first public disclosure of your invention, but it is too early to file if you cannot explain how the invention will be implemented.
First To File Explained
Under current U.S. law (as of the 2011 America Invents Act), patents are awarded to the first inventor that files for patent protection. This differs from the manner in which United States law used to operate, which would award the patent to the individual that first invented the invention, even if they were not the first to file for patent protection.
Because the United States now operates as a “first-to-file” country, it is important to file for patent protection as soon as possible in order to file your application before anyone else files on the same invention. You should know that it is not legal for someone who did not invent anything to learn about your invention from you and then race to file before you file your patent application. This is because only an "inventor" can file for patent protection, and if someone steals the idea from you, they are not the inventor. Nonetheless, the best way to avoid this scenario and all of the complicated issues it creates is to file your patent application before you tell others about your invention. For instance, it is almost always best to have a patent application on file before you begin to discuss the invention with possible investment partners or with any companies that may be interested in making or acquiring your invention.
Deadlines for Filing a Patent Application

Under U.S. patent law, you must file your patent application within one year of the first offer to sell your invention, or within one year of your first public use or disclosure of your invention. This means that you must determine the first offer to sell date, or the first public disclosure date. If it is more than one year from that date, it will be too late for you to file a patent application (See Bitlaw's legal discussion on the novelty requirement for more information). In the above chart, this means that you cannot file for patent protection more than one year from that first public disclosure.
Patent attorneys frequently encounter inventors that need to immediately file a patent
application in order to have their application meet this one-year deadline. In some instances,
it may be necessary to file a
No Need to Physically Implement Your Invention
You do not have to actually make or implement your invention before you file for patent protection. Many valuable inventions are protected by patents even before the first working prototype is constructed. However, you are allowed to file for a patent application before implementation only if you can describe your invention in sufficient detail that a person having ordinary skill in your technology is able to create your product “without undue experimentation.”
This requirement is known as the enablement requirement. If you do not know how your invention can be implemented (i.e., you do not think that the invention is easily implemented using current technology), then it is too early to file for patent protection. You may be able to physically file the application, but it could be subject to rejection from the Patent Office or a later validity challenge for failing to enable your invention. Additional research or experimentation is required before the idea is considered inventive enough so that a patent application can be filed.
In the context of the above chart, it is not enough to simply conceive of an anti-gravity device to move heavy objects, you must know how such a device could actually be implemented. Once you know how to implement it, you can file a patent application that describes the invention in such detail that another person of ordinary skill can implement your invention. It is not necessary to actually build your device since you can file for patent protection before or after you implement your new invention.
You Should Complete the Inventive Process before Filing a Non-Provisional Application
As explained immediately above, you do not need to build a device before you file for patent protection as long as you can describe the invention sufficiently to enable someone else to make the device. Nonetheless, it is important not to file the application before you know your invention will work. If your invention is a physical device, and you are not positive that the device can be made the way you envision it, then it may be wise to attempt construction of the device before spending your money on a non-provisional patent application. As explained in the guidance provided on Should I file more than one provisional patent application?, it is possible to file multiple provisional applications before filing a non-provisional application if you are still inventing new aspects of your invention.

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Guidance on Filing A Patent Application
These pages provide guidance on filing a patent application with the United States Patent and Trademark Office.
What is a patent application?
What rights are created by a patent?
What if you cannot afford to enforce your patent rights?
What are the benefits of patent pending status?
What makes patent application so expensive?
What is the cost breakdown of a patent application?
What is the cost of a design patent application?
Can't I get a cheaper patent somewhere else?
How to identify your invention (not my proposed product)?
How to value your invention
Should you trust invention submission companies?
What difference do the "first to file" laws make?
What are the deadlines for filing a patent application?
Do you need to create a working model of your invention?
What is a patentability search?
How can searches avoid wasteful patent applications?
How can searches improve a patent applications?
When is a search unnecessary (or even unhelpful)?
What is a provisional application?
Do provisional applications save money?
What are the benefits of a provisional application?
Can I file a provisional application without an attorney?