You're not allowed to sleep, you need to take control within 10-15 seconds. And no, it doesn't turn off if there are no vehicles in front, but it does turn off

You're not allowed to sleep, you need to take control within 10-15 seconds. And no, it doesn't turn off if there are no vehicles in front, but it does turn off above 45 MPH. On most highways you'll only be below 45 because of traffic. Above 45 they have a separate L2 system that requires "normal" attentiveness.