clean-buffer-list-delay-general

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

3

Documentation:
The number of days before any buffer becomes eligible for autokilling.
The autokilling is done by `clean-buffer-list' when is it in `midnight-hook'.
Currently displayed and/or modified (unsaved) buffers, as well as buffers
matching `clean-buffer-list-kill-never-buffer-names' and
`clean-buffer-list-kill-never-regexps' are excluded.

You can customize this variable.