How long should kids play outside every day?

Until they finish . If your children are in a safe environment let them play as long as they wish or until the street lights come on :-).
As much as they want. Try to get them outside at least 15-30 mintues every day. Vitamin d levels will do best with at least 15 minutes of sun every day. Beyond that its all good exercise and fesh air with good skin safety and sun protection. Encourage lots of outdoor play if they wish and more if they tend to be indoor play people.