VMS Help SET, VOLUME, Qualifiers, /RETENTION  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /RETENTION=(min[,max])

    Specifies the minimum and maximum retention times to be used by
    the file system to determine the expiration date for files on
    the volume. When a file is created, its expiration date is set
    to the current time plus the maximum time. Each time the file is
    accessed, the current time is added to the minimum time. If the
    sum is greater than the expiration date, a new expiration date is
    computed.

    If you omit the maximum value, a default value that
    is the smaller of (2 x min) or (min + 7) days is
    used. For example, /RETENTION=3- is the same as
    /RETENTION=(3-,6-), while /RETENTION=10- is the same as
    /RETENTION=(10-,17-).

    The command SET VOLUME/RETENTION=0 disables retention times on
    the volume.