You should only water when the soil is dry. So you won't necessarily be on any type of schedule. For me, it can be as much as every day, or as little as twice a week, depending on the rate which the plants take up the water, and how fast the soil drains out. For seedlings you want to be sure you don't give them too much or you will kill them. Usually when the top 2 inches of soil are dry, its safe to say they are pretty thirsty and would like a drink.