At /usr/share/kodi/system/setting/settings.xml the GUI definitions and default values are stored. They are read-only by concept.
Unfortunately <maximum> is part of the GUI definitions, and not editable by overwriting it at /storage/.kodi/userdata/guisettings.xml.
PS: It should be possible to write a slideshow as a Python script. Then you can set the pause as long as you want.