You never want to over-water. Wait until the soil is DRY, like crusty dry on top, to water.
Cannabis is a weed, you must treat it as such. I know it's hard not to baby them, but it really is for the best. Just let them do their thing, and water when it needs it.
About how much to water, I like to water until I see some drain out of the bottom. My girls in 5 gal buckets get about 1-2 liters every 5-6 days.