Correction: 24 plant RDWC SoG
There will be up to 30 gallons in the bottom res. The top ones will hold about 8 gallons each since they won't be filled to the brim. If the bottom and the top reservoirs are filled, it won't run out of solution. When the water level in the top reservoirs reaches the water line, they will drain into the lower res. The water just recirculates.
15 minutes every hour sounds like it would work fine. That would refresh the water in the upper reservoirs once an hour