What is happening in lautering is diffusion of sugar-heavy liquid into less-dense water. You should balance run-off rates versus efficiency and come up with something acceptable for your situation. If you could run off in 10 minutes, big whoop if you are leaving all of your extract behind in the lauter. A 20 minute longer runoff might be a percentage increase in efficiency, which will save you money in grain in the long term. Or if you are pressed for time, you might be willing to give up a percentage efficiency to turn around the lauter tun faster. It all depends.