Well, normally four hours would be plenty - my typical long ride takes something over 3 hours. When I finish breakfast and ride home, it's light.

But there will be times I'll be riding even further, such as riding around the whole island (just over 100 miles if I follow the main roads). I'll still be leaving real early - probably before 2:00 AM for such a ride. Sunrise can be as late as about 7:20 so four hours would not be enough. And that's a LOT of money for these powerful lighting systems!