set ubuntu screen saver idle time

After searching for a very long time, I got some answer at last:

first method is to use gsettings

gsettings set org.gnome.settings-daemon.plugins.power sleep-display-ac 600
gsettings set org.gnome.settings-daemon.plugins.power sleep-display-battery 600
gsettings set org.gnome.desktop.session idle-delay 600
gsettings set org.gnome.desktop.screensaver idle-activation-enabled 'true'
gsettings set org.gnome.desktop.screensaver lock-enabled 'true'
gsettings set org.gnome.desktop.screensaver lock-delay 900

another way is:

vim ~/.gconf/desktop/gnome/%gconf.xml

<?xml version="1.0"?>
<gconf>
    <entry name="idle_delay" mtime="1282047258" type="int" value="2"/>
</gconf>

which indicate default idle time is 2 minutes. Interesting!