Other people my have better answers... but here is mine:
It shouldn't matter how long they play... and i would rather some of that puppy energy get used up on another puppy/dog then my <blank> (insert: shoes, table legs, pillows, couch, kids xbox controllers, drywall, landscape... etc).
I also think playing is an important part of being a puppy. It teaches the puppy limits (whats a good play nibble vs. bite) and social skills. Puppy's start to learn this with litter mates, but when you bring them home they don't have anybody to test the limits with any more (other then you and your possessions).
My Molly (8month old) spent all Saturday playing with our other dog and the neighbors dog.. She spent most of sunday sleeping and just lounging around the house. My older dog tried a few times to get her to play... but she was pooooped.
The exception to the rule: It seems EB's get more wound up the more sleepy they get... Molly goes spasmoe-crazy if she gets super tired. We sometimes have to pull her off our other dog and put her in her crate at night. She'll start snoring before the crate door is even closed.
As for the puppy class 'aggressive' behavior.. Bulldogs seem to play at a 'larger' dog level (puppy EB + puppy shih tzu = trouble, puppy EB + puppy lab = broken furniture).