This depends. The first few showers post application may have some washout, it’s normal until you shampoo for the first time, and that’s similar to retail chemical brands. Other factors are more specific: application process, existing hair health, previous coloring procedures, and so on. Healthy hair will bleed very little after the first washing. Henna, Indigo and the accompanying herbs like to bind to healthy hair proteins, on smooth naturally lubricated shafts of hair. If you have damaged hair, ravaged by chemical processes—prior to your henna treatment, then you may have more color washout vs. a head of healthy hair. Your protein(keratin, etc.) simply require more time to restructure and restore proper pigment retention abilities. Understand: you will still have pigment absorption, but it will take time for max pigment uptake. As you apply additional henna treatments, and as your hair breaks away from the chemical baths of before, your hair will bleed less and peak faster in pigment retention.
Tip: use old dark colored towels for the first few showers (as precaution). Henna & indigo can stain fabric, but most heavy stains will come out in wash. Straight, Pure Henna will have the least amount of washout.