Shorter lounge line means a smaller circle, and smaller circles are harder on young joints.
My opinion is let your horses decide how much work they do. If they want to play and use a lot of energy, let them, use it to your training advantage, but don't ever push them beyond what they want to do. Also, if you get the feeling they are over doing it, ask them to do slower exercises, like neck flexions and basic yielding.
Keep each session to an hour or less. IMHO, a young horse should NEVER drip with sweat after a lesson, I let my babies decide how active or how slow they want each lesson to be as long as they keep it within reason.