How often you apply Hairprint depends on your objective.
The Hairprint color will typically last about two to six months. This time frame can range depending on your type of hair, how often you shampoo, what you shampoo with, exposure to salt water, chlorine and sun. If you do not have predominantly gray hair, you may find applying every few months will be fine for you.
If you have a higher percentage of gray and would like to continuously cover gray roots to avoid them being seen, you will need to reapply as often as your hair requires, generally from 3-5 weeks depending on the rate of your hair growth.