Charlotte Mason advocated short lessons for a young child. Lessons would then become progressively longer as the child matures. Elementary-age children's lessons would be no longer than fifteen or twenty minutes on one particular subject before moving on to the next thing.
With my ADD children, lessons were sometimes even shorter. Ten minutes when they're little and then gradually longer as they get older. Sometimes, a few minutes on the swingset or building LEGO's were inserted in between lessons. (I'd have to use a timer, lest I forget to call them back in to work!)
The idea is to gently encourage the habit of full attention over time, while exposing them to a variety of subjects and activities.