There are many little factors to consider when deciding how often to water. How much soil there is, how fast it dries, how humid the air is, how big the plant is, how fast you want the roots to grow and other things affect watering frequency so it's hard to give a simple rule of thumb for everyone. Ideally, you want the soil to dry out almost completely between waterings. The plant gets a little growth spurt towards the end of drying because the roots are getting alot more air. The roots will also grow faster at this point so the plant won't be able to stay as long in the current pot. I'd say that usually once the top, i dunno, maybe 1/3 of the soil is bone dry, it's safe to water. The plants can also survive the soil reaching complete dryness (not reccomended), depending on the conditions. You'll have to decide what you like best for your babies.
