Basically run the lights to meet and level out your nuitrent level. Some people do run them 24/7 to keep up with their nuitrent production or try to. Best to give the algae a rest period we are told.When the light is on photosynthesis is occurring and PO4 & NO3 are assimilated into the algae, i.e. removed from the water. If you want PO4 & NO3 levels to be higher have the light on less than 24/7.
It's not the norm to run a scrubber lights 24/7. You can run them just 7 hours a day. It all depends on what level you want to keep NO3 & PO4 at.