how to change screencast duration in gnome3 screen recorder

1. Open the dconf-editor
2. Find org.gnome.settings-daemon.plugins.media-keys in the tree of settings
3. Find the setting named max-screencast-length (the default value is 30 seconds)
4. Change it to 600 seconds for 10 minutes (10 * 60 seconds), or 1800 seconds for 30 minutes (30 * 60 seconds)

Open a terminal by pressing Ctrl+Alt+T and type:

gsettings set org.gnome.settings-daemon.plugins.media-keys max-screencast-length 1800

Which will set the maximum screencast length to 1800 seconds, which is what you want! ;-)