As long as they choose to play
The activities children engage in should be partly of their own choosing; they should have the time, space, materials and freedom to play, create and imagine on their own terms. Teachers and other adults should not be the sole providers or initiators of what children "need" to experience.
The ultimate criteria for success should be the increased happiness and involvement of the children, not help for working parents, not tackling obesity and not providing extra study support for already over-tested children.
Professor Colin Richards 1 Bobbin Mill Spark Bridge, Cumbria