clean-buffer-list-delay-special

clean-buffer-list-delay-special is a variable defined in `midnight.el'.
Its value is

3600

Documentation:
The number of seconds before some buffers become eligible for autokilling.
Buffers matched by `clean-buffer-list-kill-regexps' and
`clean-buffer-list-kill-buffer-names' are killed if they were last
displayed more than this many seconds ago.

You can customize this variable.