Watering Sponge Cubes

I have been rooting some clones in these little sponge cubes. About 1.5 inch in diamiter. They are rooting under a 600 watt MH light. On a rule of thumb, how often should I water the cubes. I am currently watering by hand from the top. Many are rooted through the cubes and many are not. I am currently watering 1 time a day. Should I be doing this more often.?Thanks

I just keep a little water in the bottom of the tray