Outdoor lights timer: Intermatic HB851 photo timer with dial. How do I set it for dusk ON then off in 8 hrs.

Intermatic make HB51 timer.

If this is not your timer, then add a comment.
To set for ON at dusk, and off 8 hours later, rotate dial to the number 8.
Timer turns on automatically at dusk.
Timer set to 8 hours will start countdown when timer turns ON > and 8 hours later timer will turn off.

Make sure timer is turned away from lights so photo-eye doesn't receive feedback.
Feedback is when the photo eye gets too much light, and then timer thinks it's dawn, so lights turn off, and a moment later photo eye thinks it's dusk again and lights turn on again.
Feedback from lights will cause timer to reset the 8 hour countdown.
A car pulling in the driveway can cause feedback.
If the timer resets because of feedback, it begins the 8 hour countdown again, so the lights will not turn off until dawn.

You can test timer by putting black tape over photo eye, and then timer will turn on.
Set timer for 2 hours for example and then check if timer turned off 2 hours later.

If timer is not functioning correctly, then timer is defective.