Give children aged from 3 to 6 years old 10 minutes for an explanation and 20 minutes for an exercise.
Their ability to concentrate has increased by this age, but in a non-linear way: depending on whether the activities require a lot of intellectual input or not, the time that children are able to stay engrossed in an activity will vary.
In class or at home (for example, when they're doing their homework), 10 minutes is the maximum time you should spend explaining a lesson, the rules of a game or an activity. Any more and there's no point in trying any harder: your child will get distracted, stop listening, wriggle around on their chair and start fidgeting.
Take a short break and then move on to the exercise. Whatever the activity (writing, drawing, music, playing outside with others outside, etc), let it continue for 20 minutes.