A primary rule of the patent system is that you should not tell anybody about your invention before you have filed a patent application. An invention must be novel (new) at the date on which a patent application is first filed for the invention. Novelty is judged against the ‘prior art’, which is all information made available to the public before the filing date of the patent application. If you've already publicised your idea, then it is no longer novel. Publication includes any disclosure of the invention without a confidentiality agreement.
However, many countries provide for a "grace period" during which any disclosures of the invention are not counted as prior art. For example:
Albania: 12 months from the priority date
Armenia: 12 months from the priority date
Argentina: 12 months from the priority date
Australia: 12 months from the national date
Bulgaria: 12 months from the priority date
Brazil: 12 from the priority date
Belarus: 12 from the priority date
Canada: 12 months from the national date Germany: 6 months from the priority date (Grace period in Germany applies only to utility models, not standard patents)
Ecuador: 12 months from the priority date
Estonia: 12 months from the priority date
Georgia: 12 months from the national date
Japan: 6 months from the national date
South Korea: 12 months from the national date
Kazakhstan: 12 months from the national date
Lithuania: 6 months from the national date
Moldova: 12 months from the priority date
Macedonia: 12 months from the national date
Mexico: 12 months from the national date
Malaysia: 12 months from the national date
Peru: 12 months from the priority date
Philippines: 12 months from the priority date
Romania: 12 months from the priority date
Russia: 6 months from the national date
Saudi Arabia: 12 from the national date
Turkey: 12 months from the priority date
Taiwan: 6 months from the national date (the grace period in Taiwan is presently limited to disclosures that are ‘not intended by the patent applicant’ and ‘printed publications’, which encompasses more than unauthorised disclosures, but is not completely unconditional)
Ukraine: 12 months from the priority date
USA: 12 months from the national date
Note that the European Union has no period of grace.
The "priority date" is the grace period ending on the date of filing of a national application in the country in question (typically this requirement can also be met by filing an international application).
The "national date" is the grace period ending on the date of filing of the first application, in any country, from which priority can subsequently be claimed in a national application.
Only disclosures by the inventor, or a subsequent owner of the invention, are allowed the grace period. This can include indirect disclosures, such as where the inventor discloses the invention to a third party, who subsequently makes a separate public disclosure. Disclosures made by an independent inventor who happens to come up with the same, or similar, invention remain available as prior art against a patent application.
Relying on a grace period can result in a reduction in the patent rights. In Australia, there are ‘prior user’ provisions which mean that if a third party learns of an invention, and commences exploiting the invention themselves during a period prior to the filing of an application, that third party can continue to exploit the invention even after a patent application is filed and granted. Similar prior user rights exist in other countries.
Japan requires applicants to declare any prior disclosures at the time of filing in order to obtain the benefit of a grace period.
Paragraph 24(1)(a) of the Australian Patents Act 1990 provides that:
(1) For the purpose of deciding whether an invention is novel or involves an inventive step or an innovative step, the person making the decision must disregard: (a) any information made publicly available in the prescribed circumstances, by or with the consent of the nominated person or patentee, or the predecessor in title of the nominated person or patentee … but only if a complete application for the invention is made within the prescribed period.
Regulation 2.2C of the Patents Regulations 1991 defines the relevant ‘prescribed circumstances’ (essentially, any circumstance not otherwise provided for specifically), and the ‘prescribed period’, which is ‘12 months from the day the information was made publicly available.’
In most jurisdictions, you should not rely upon the use of any grace period that may be available. It is always preferable to file an application sooner rather than later.