How long should I keep original non-disclosure agreements on file?

With the easy ability to scan and digitally store these documents, there's really no reason to ever not maintain a digital copy of these documents on a hard drive and/or on a CD-R or DVD. As for the physical copy, if the document has expired, you can get rid of any time you want.

Whose information does the agreement protect. If it is the client's, you better have them available for litigation if your company unintentionally breaches. Best of Luck!