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It depends on the number of fields, and the size of the fields. These are defined in the message (http://docs.ros.org/api/sensor_msgs/html/msg/PointCloud2.html) fields[] array (http://docs.ros.org/api/sensor_msgs/html/msg/PointField.html).

If you're doing something quick and dirty and you know the shape of your incoming data, you can always hardcode the sizes, however the more robust approach would be to process based on the fields[] data.