It really depends on your rotifer cultures. If you have LOTS of rotifers and won't run out, then you can light the tank for longer. If you start to run low, go with a shorter photoperiod.
I would leave the tank lit (dimly with blacked out sides) for the first 18 hours they are alive. This can help them get a good jump on learning how to hunt.
Then, I would go with a 16 on and 8 off cycle.