The American Academy of Dermatology says that hair grows about 1/2 inch per month on average. That’s about 6 inches per year. The next question is how radical us your haircut.
If you have really short hair and your undercut is of a softer design, it should only take you a few months to grow out. On the other hand, If you have very long hair, it can take up to two years for lengths to match up.