I've had success at all hours. However, here is something that I noticed last season...
As soon as it turns dark to the point where if you did not have a light system (and you would have to go home as a result)the fox really seem to move. Even after an unsuccessful dusk calling set up. I stay put. When darkness sets, I wait a few minutes then hit the call again. It's amazing how many times the eyes glow and the fox come right in. If I had no lights, I'd be heading for the truck and missing out on some hot calling action.
This being said, I also score a lot between 10:00 PM and 2:00 AM.
I really believe that any time you are out there and a critter is in the vacinity, he will check you out and then THAT IS THE MAGIC TIME!