Unlike higher plants that require on/off cycles, algae has the capacity to use light 24/7.
If the scrubber is in a sump, or stand alone, the light can be simply left on.
But you could experiment to see what species of algae is produced by your own conditions, or seed the scrubber with the most desirable species you can find.
I prefer hair/filamentous algae, and am lucky it simply appears naturally, and grows great in one of my tanks, so I seed my scrubbers with it.
And because some of my fish eat it, when I pull off excess, it is recycled.