How long does a patent protect a technology?

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.