Recently I have discovered that 'Cheese' webcam application does not allow to change photo countdown duration through its 'Preferences' menu. I have been pleased to find out that dconf-editor has org/gnome/cheese section with 'Do not use default' slider switch and editable countdown duration field. Well, it occurs that maximum duration is limited to 10 seconds and that limit is not editable in dconf-editor.
Fortunately, it is possible to change that limit.
# cd /usr/share/glib-2.0/schemas
# sudo nano org.gnome.Cheese.gschema.xml
...
<key type='i' name='countdown-duration'>
<summary>Countdown length</summary>
<description>The duration of the countdown before taking a photo, in seconds</description>
<default>3</default>
<range min='1' max='30'/>
</key>
...
# sudo glib-compile-schemas /usr/share/glib-2.0/schemas/
With dconf-editor relaunched and 'countdown-duration' key reread it is possible to set a desired delay beyond 10 s limit.
Enjoy!