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Each message in the /velodyne_points topic contains all the data from a complete revolution of the device.

If you write a ROS node that subscribes to that topic, your subscription callback will be called each time a new message arrives. That happens at 10 Hz with the device spinning at the default 600 RPM rate.

Here is an example node that reads the point cloud for each revolution, looking for points within some grid cells that significantly differ in the Z axis.

You can figure out from that how to write your own subscriber node.