For soil grows, you generally only want to water the plant when the soil feels dry if you press a finger in it. For other growing mediums besides soil, your watering method will vary, but if your plants are drooping then you may be over- or under-watering them.
While it can be good to let your plant dry out a bit in between waterings, you should always water the plant again before it's leaves start drooping from thirst. A good way to tell if a potted plant is ready to be watered is to pick it up and tell if it feels heavy or not. As plants use up all the water in their pot, it will start to get lighter. If you need something for comparison, you can get an extra pot and fill it with your growing medium. Now you can use this extra container for comparison with your potted plants as it represents the 'dry weight' of your growing medium. If you pick up a potted plant and its feels just slightly heavier than your dry pot, then you know it's time to water your plant. After a while you get a feel for how heavy your plants need to be and you may not even need the extra pot anymore.