How long does a patent protect a technology?

A patent lasts for 20 years and the duration of the patent is determined from the date of filing of an application. There is no provision for the original moment of the invention although it is prudent for you to keep your own records if issues around ownership were to arise.

The UK IPO provides lots of free information on patents - accessible on https://www.gov.uk/intellectual-property/patents

Patent term is normally 20 years from the filing date of the application. The 20 year term runs from the filing date, not the priority date, of the application. For a national phase of a PCT application, the filing date is taken as the filing date of the PCT application.

Patent term in the US can be extended for longer than 20 years if the USPTO has delayed responding to an application request for patent, exceeded 3 years to consider a patent application, or due to a secrecy order or appeal.

For US patents filed prior to June 8, 1995, the term of patent is either 20 years from the filing date of the earliest application in the patent family (excluding provisional applications) or 17 years from the date of grant, whichever is longer.

In members of the EU, if the patent relates to certain regulated active agents, such as human or veterinary medicaments and plant protection products, then additional protection of up to 5 years for that product can be obtained via an Supplementary Protection Certificate (SPC). The extra protection is for the particular product (i.e. active agent or combinations thereof); it is not an extension of the patent term over the entire claim scope. Six additional months of similar protection is available for paediatric medical products. Similar additional protection is available in certain other jurisdictions, e.g. Japan.

So, patent term runs from the filing date, not the grant date. The period between the devising of the invention and filing the application does not affect patent term.

Ideally, the term of enforcement of a U.S. patent is for 20 years from the earliest non-provisional application filing date. I say ideally because time consumed to prosecute the patent application to allowance "eats into" this 20 year term. So the sooner you get your patent application to allowance, the more of the 20 year term your enforceable patent will entail. While the patent application is pending, there are no enforceable patent rights, although there is something known as "provisional rights (PR)". This means that if your patent application is published (typically 18 months after the earliest filing date) and your patent application is eventually allowed and issues into a patent, AND IF the claims of your published patent application are substantially identical to the claims of your issued patent (which is usually NOT the case), then it may be possible to collect a reasonable royalty from infringers as far back as the publication date of your patent application. With regard to the "original moment of invention", if that is means an inventor reduced his invention to practice before he/she filed the first patent application, under the new America Invents Act, that date is not considered as part of the 20 year term.