Ok, maybe the problem here is that this time is added to the time you specify on the grasp (pre_grasp_posture and grasp_posture).
So I put on my grasp message:
grasp_data.pre_grasp_posture.points[0].time_from_start = ros::Duration(3,0);
grasp_data.grasp_posture.points[0].time_from_start = ros::Duration(4,0);

So I'm expecting to wait for 3s when doing the pre_grasp_posture, and for 4s for the grasp_posture

But on execution the goal sent to the gripper controller (a hand in our case) is (for the pre_grasp_posture):

   joint_names: ['hand_right_index_joint', 'hand_right_middle_joint', 'hand_right_thumb_joint']
    points: 
      - 
        positions: [0.10000347159257128, 0.10000347318852754, 0.09998135899197091]
        velocities: [0.0, 0.0, 0.0]
        accelerations: [0.0, 0.0, 0.0]
        effort: []
        time_from_start: 
          secs: 7
          nsecs: 0
      - 
        positions: [0.0, 0.0, 1.3]
        velocities: [0.0, 0.0, 0.0]
        accelerations: [0.0, 0.0, 0.0]
        effort: []
        time_from_start: 
          secs: 10
          nsecs: 0
  path_tolerance: []
  goal_tolerance: []
  goal_time_tolerance: 
    secs: 0
    nsecs: 0

This is set here: https://github.com/ros-planning/moveit_ros/blob/hydro-devel/manipulation/pick_place/src/plan_stage.cpp#L88
pre_approach_traj->addPrefixWayPoint(pre_approach_state, PickPlace::DEFAULT_GRASP_POSTURE_COMPLETION_DURATION);

My first focus is to make this work on my robot, so changing that to 1.0s made the trick. I see it's only used for opening and closing the gripper. Shouldn't then the time_to_start field of a grasp be checked somewhere and if it's 0 then add this default value? Cause right now it seems it's adding it all the time.

Sorry if I'm not so clear explaining myself! But I'm tinkering around a lot for making the pick work (later on the place... and pick and place I hope).