I'm assuming you mean hydroton pebbles, will each plant be in seperate pots or one huge table? I grow with hydroton in seperate pots for each plant and generally water my plants 3 times a day some times only twice a day and have yet to see any problems of plants drying out, of course if in one huge table it may dry out a little faster so maybe 4 times should be enough, just keep an eye on them, they will tell you if they're hungry or not.