/sys$common/syshlp/helplib.hlb Sys Parameters, TTY_TIMEOUT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    TTY_TIMEOUT sets the number of seconds before a process
    associated with a disconnected terminal is deleted. The default
    value (900 seconds) is usually adequate. Note that using values
    for TTY_TIMEOUT greater than one year (value %X01E13380) can
    cause overflow errors and result in a disconnected device timing
    out immediately.

    TTY_TIMEOUT is a DYNAMIC parameter.