Hi Jean-Philippe,
Your question is not easy to answer in a few words because the subject is quite complicated.
Depending of the equipment of the airport, the present weather, the type and the thickness of the contamination, the de-icing process can be conducted at the gate or on a special zone of the airport.
For example, to de-ice an aircraft covered with morning frost while the weather is cold but fair, it will take 10 to 15 mn at the gate, before engine start.
For an aircraft covedred with a thick amount of snow or ice, during an winter weather with heavy snow and wind, it will need to proceed in two phases, first de-icing to eliminate the contamination and then, protection with ant-ice liquid. In this case, it will take 20 to 30 mn on a de-icing zone, engines running or not, depending of the airport equipment and regulation.
Michel.