Hey stunzeed, Hick's said it all man. I hear people all the time saying "I give them exactly 500ml of water every other day". Using a measured amount of water on a schedule for a marijuana plant is nuts.
During it's growth, your plants will need varying amounts of water. The usage, of course, will increase as the plants grow larger and radically increase during flowering.
If you water by weight, you can't go wrong. Get a good quality DIGITAL bathroom scale. Let your plants go without water until you see a slight droop to the leaves. Lift the plant in your hands. The container should feel light, like there isn't any dirt in it. Now, weigh the plant and container. Write down the weight, date and time. Now, subtract 24 hours from the date. That's when you should have watered them to be perfect.
Water the plant, as Hick said, using a slow pour to saturate the soil and root system. Do so in relatively small doses so that the water won't leak out the bottom until the dirt and root mass is saturated. Put a plastic cover over your scale to catch any water that may leak from the container during weighing. Look to see what the scale reads without the plant on it. Now, put the plant on it and write down the date, time and how much water the plant took and subtract any weight the plastic had, so you'll know how much water weight you added.
When it is 24 hours from the "dry" date, weigh the plant again. Write this weight down with the date and time.
Water the plant as before. Now, just water when you reach the point you were at when you watered 24 hours before the "dry" date. The plant will still have enough water to prevent it from having shock, and you'll never over-water or underwater again. Using this system will allow you to closely monitor your plants water usage as well, and will give you a better understanding of how your plants health is.
If the plant has any radical decrease in water usage, it may have a problem that needs attention.
Good luck to you man!