Certified GKH Gardening Expert
Water only when the soil is dry to the touch (when your finger feels dry soil 10 cm down or so). Allow the plant to dry between waterings.
Dry soil will not stick together when you pinch it between your fingers; however, it should not be hard and scratchy, either. And test the soil as close to the bottom of the pot as you can get; yuccas should aerate all the way to the bottom.