As a rule of thumb, a 20% tip is generally considered appropriate for any service, including a haircut or shave. This ensures that the barber receives a fair and generous compensation for their work. However, tipping can vary depending on a number of factors such as the quality of the service provided, the duration of the visit, and the personal relationship between the customer and the barber. Ultimately, it's up to you to decide how much to tip based on your own circumstances and personal preferences.