Hi Philippe,
In the commands below I use 29 as an example value but you can ofcourse fill in any value you want:
To set the time before display to sleep when on AC
gsettings set org.mate.power-manager sleep-display-ac 29
To set the time before display to sleep when on Battery
gsettings set org.mate.power-manager sleep-display-battery 29
the time before screensaver trigger:
gsettings set org.mate.session idle-delay 29
It is not that I know these things spontaneously: I used dconf-editor's search to look up the name of the schemas and the name of the variables.
I don't know if this is considered cheating but it does it come in handy at times 