I am trying to figure what's the best way(standard) to detect when all box2d objects stop colliding. For example if I drop a box2d body onto a pile of body, how can i detect when all the collisions ended (or settled down)
You mean when they stop moving right? That's not the same as 'stop colliding' because they will still be colliding with each other if they are piled up.
How about checking the linear velocity of them, and if it is less than a threshold value you could consider the pile to be settled.