If you are using the to supplement the MHs then run them one hour before the MHs turn on until one hour after the MHs go off. The lunar lights can be ran any way you want. Some run them for a few hours at night for viewing pleasure and then have them shut off and other run them all night.
Now, the reason I say there is no right answer is because people run their lighting longer and shorter than 8 hours and it works well for their systems. So don't take the 8 hours for gospel, it's just what I have had success with. You could run them longer if you want.