Frequency of watering and how much you apply depends on your soil type, and climatic conditions in your areas. For that reason, one cannot provide you an accurate recommendation on how often to irrigate or how much to water to apply. Soils in a field often vary with each having different requirements for irrigation amount and frequency. Growers can fine-tine their irrigation needs if they know what soil textures are in their field.
Lavender Irrigation: Determine the soil texture:
You may already know the texture of your soil, if not Web Soil Survey (usda.gov) provides the best possible update on the soil textures of many fields. Not all areas of the United States have been surveyed but hopefully your fields have been surveyed.
Enter your address and click on view. Select AOI to outline the area you are interested in and click on Soil Map. This will show you the soil texture and any specific properties and qualities of the soil you are working with.
Texture changes with amendments:
While the Web Soil Survey may not be completely accurate it is a great place to start. Keep in mind that as you amend the soil with organic matter you will be increase the infiltration rate and water holding capacity of the soil but knowing the texture will be a great starting point. The following chart gives more detail.
The Managed Allowable Depletion % shows how much water can be safely removed from the soil prior to your next irrigation. Removing more than recommended can result in plants suffering from lack of water (Permanent Wilting Point).
The Infiltration rate tells you how fast you can apply water. If using drip or micro spray irrigation, this determines the application rate of your emitters. If you apply water faster than the infiltration rate you will end up with puddling of water on the soil surface and possible suffocation of roots.
Determine the soil moisture with the soil ball technique:
You can use the soil ball technique to finetune your irrigation as this will give you an approximate percentage of moisture in your soil. https://www.wcc.nrcs.usda.gov/ftpref/wntsc/waterMgt/irrigation/EstimatingSoilMoisture.pdf
Keep in mind newly planted lavender will need more frequent irrigation as the roots are still quite shallow and restricted to the root ball area. Checking the moisture content of the root ball with your index finger is helpful in determining how frequent to irrigate. Once roots spread into the surrounding soils your irrigation scheduling will be much easier.