Lidar is collected per PhysX step, at a single timestamp, The slight offsets for the scan that occur during a step are currently not taken into account.
The amount of data (number of rays) returned is based on the rotation rate of the lidar as well as the framerate of the simulation. If the rate is faster and the framerate slower, the scan should be larger. All of the rays computed during that physics step for the scan would have the same effective timestamp.
We are working on exposing more apis for RTX lidar (And possibly the PhysX range sensor lidar) that will provide per ray timestamp information. But currently its not computed for the range sensor extension