I usually only spend 30 to 45 minutes depending on the bite. The fish move along the drop offs like you described looking for bait fish like they do during the day. Even if you see bait fish in the light the majority of bait fish may have move as well and so did the crappie.
The crappie could also be inundated with all the bait and you need to drop your lines to the right spot. I will turn my light off after 30 minutes to see if the bait fish get "hit" top water in a frenzy and turn the light back on real quick. This gives me an idea if there are still fish below feeding. If the bait fish do not swarm and you can't hear the fish hitting the bait on top all over with the light off then there are no predator fish around.
I will move after 45 minutes to an hour if no bites what so ever. Lots of drop offs and other timber to explore and catch them.