/sys$common/syshlp/helplib.hlb MONITOR, MONITOR, Command Qualifier Descriptions, /FLUSH_INTERVAL  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /FLUSH_INTERVAL=seconds

    Specifies the interval, in seconds, at which data collected by
    MONITOR (contents of MONITOR buffers) is written to disk. Values
    must be in the range from 1 to 9,999. The default interval is 300
    seconds.