Water until it starts to leak out the bottom into the tray, I normally stop watering when this happens and I get about 1/2 or 1/4 in of water in the tray, this is water is removed quickly by the plants.
Its important to water your plants in stages. Though I normally put a about a gallon per pot per water, I start by adding 1 gallon to all six eyeballing the amount. I wait a few min, water again, wait a few water again.
The reason I do this is because soil tends to develop dry pockets that repel water, if you water all at once the water rushes by these dry pockets and runs down into your tray, this might give you a false positive as far as watering enough. Something to be aware of.
Also, aerate your soil, I poke holes with my moisture meter, helps water drain evenly into the soil and lets more oxygen reach the roots.