This should do the trick, sets it to 45min:

gsettings set org.gnome.desktop.session 'idle-delay' 2700

Thank you very much, it worked ! Wouln't it be a good thing to have this feature enabled on the GUI ?

    Bouquins We're not the developers of "Settings". It's GNOME Control Center, developed by GNOME. The feature request should go to them 🙂

      You can do it graphically, IMHO the way @JoshStrobl is faster. Install and open dconf-editor, go to /org/gnome/desktop/session/idle-delay and change Other value