Lee said each trip, 30 seconds is spent loading/unloading. It wasn't clear whether he meant 30s total per trip (15s each), or 30s each for loading and unloading.

Other pertinent information is that ore production values are per hour but ore is produced every 5 minutes, and harvesters will wait at a planet of no ore is present but will leave as soon as there is something to transport, not waiting to fill.