In the context of virtual museums, “long-term” is not defined as a specific period of time. Also in the context of Digital Preservation, it is not strictly defined:  in (Verheuil 2006: 20) it means "five years or more", in (Research Councils UK 2008:6) "Data should normally be preserved and accessible for not less than 10 years for any projects, and for projects of clinical or major social, environmental or heritage importance, the data should be retained for up to 20 years, and preferably permanently within a national collection, or as required by the funder's data policy."

More generally in (CCSDS 2002: 1-11) "a period of time long enough for there to be concern about the impacts of changing technologies, including support for new media and data formats, and of a changing user community, on the information being held in a repository. This period extends into the indefinite future."