The time of the appointment can range depending on the application method used and how much hair is being applied. The average timing for a full head application for length and volume can be anywhere from 3 to 8 hours. If you are in for reconstructive hair extensions (meaning, a major transformation, like from a pixie cut to mermaid-length hair), the appointment time can range from 8 hours to 20 hours, which would be done over the course of multiple days.