It’s hard to say, as it depends heavily on the growing conditions! In general, lavender prefers growing in dry to average soils. It’s drought tolerant once it’s established, but still needs to be watered while it’s young. Water newly planted lavender two times a week to ensure the soil doesn’t dry out. When your plants are established, reduce the frequency to once a week or every two weeks. Water deeply each time you irrigate your plants to encourage deep root growth.
These are just general suggestions. Hotter climates may need to water more, while cooler climates water less. The best way to tell if your plant needs water is by feeling the soil. If it feels wet to the touch, skip watering until it dries out.