First time owner of an American Bulldog (Martha), and dogs in general.
Martha's currently 4 months old, and about 8kg. If there's a way I can describe her, she's a very quiet, non-aggressive dog, as far as her time inside the house has been.
My only problem with her is that whenever I decide that we're done playing (about 10 to 20 minutes) she starts barking at my feet. This scares me since we have children in the house, and they might react wrongly, causing Martha to do something unexpected of her.
Regarding play time, we usually keep her out for most of the day, except when she sleeps, or when we have no one to watch over her.