The previous posts have all had good advice. A lot of it comes down to learning to read your plants, because how often they want water changes with their age as well as with environmental conditions. It also changes depending on how you train them. For example, my plants that are between 1-3 weeks of vegging only need water every 3-4 days, but once I start "making it rain" in week 3 they respond by putting on growth spurts and demanding more water more often. My oldest plants get a small amount (about 1/4 gallon) every day and a large drink(1+ gallon) every other day in addition to their once a week rain (which is when I run about 2 gallons through each 5 gallon pot). I've also discovered that you need a bit larger pots with this method, since the pair I have in 3.5 gallon pots HAVE to be watered every day, and sometime end up needing watering in both the morning and afternoon.
Keep in mind, the amount you water with is going to vary from day to day. Once your plants are 3 weeks into veg it's hard to overwater them, and at least once a week your going to want to give them a heavy enough watering (without nutes) that you get a decent amount of runoff out of the bottom of the pots. I personally water until I see runoff every time, but as long as you are getting all the soil in the pot nice and moist your plants will be ok. Just make sure you drain the run off pans, because you don't want the base of your pots (and your plants roots) sitting in standing water.