Two minutes is the time most often quoted by dentists, but that recommendation is somewhat arbitrary. Actually, I think the time needed differs for each person. I recommend that they be systematic and brush each surface adequately and see how long it takes them.
When I brush my teeth I start in the upper right quadrant on my back teeth and gradually work my way to the upper left. Then I do the upper insides in the same manner. After that, I usually do the same for my lower teeth. I use an electric and I allow about 3 seconds per tooth surface.Using a manual brush might take longer per surface area- lets estimate about 10 seconds per area.
You just want to be thorough and systematic job and not go over the same areas repeatedly during the same session. I do not believe that even using a manual brush it in necessary to spend more than 10 seconds on a particular area. Using the technique I have described above brush your entire mouth and see how long it takes. That is the average time you will need.