It's just whatever works for you. You are presently at 8 hours, and that's a good baseline. You need to allow for enough run time for chlorine generation, cleaner operation and skimmer operation. If the pool is clean and you are making enough chlorine, you are probably okay. Maybe you can shorten the time and see how it goes, maybe you need to run longer.
You should always operate the pump when there are swimmers in the water for better sanitation.
If saving electricity is a goal, you could probably downsize the pump to 1HP without affecting the pool, or switch to a 2-speed or variable flow pump.