A Non-Disclosure Agreement is a contract between one or more parties agree not to disclose any confidential information that they have shared with each other as a necessary part of doing business together.
A Non Disclosure Agreement is able to last up to 2 - 5 years. However it is written in the NDA that once the information is made public then the document has lost its confidentiality and the public will be able to be free and disclose the information.