Most martial arts expect a student to reach a 3rd dan black belt standard before training to become an Instructor.
This is a good guideline for tai chi chuan also.

Typically a Tai Chi Chuan Instructor has at least 5 years experience and is ideally a lineage disciple.
They have a serious commitment to home practice and clear evidence of ability in all 13 areas of skill.
The Instructor should possess a stage 2 knowledge of form and be capable of teaching different forms e.g. Yang Cheng Fu, sabre, cane, staff, walking stick, jian (or equivalents).
They should be able to apply every posture of the Yang Cheng Fu form in a wide variety of ways.

There are 5 levels of Tai Chi Instructor in the UK these days:

1. Health-only Tai Chi Teacher
   - 5 years experience
   - these are the majority
   
2. Tai Chi Chuan Instructor
   - equivalent of 3rd dan black belt in any martial art
   - 5 years experience
   - these people are less common
   
3. Tai Chi Expert
   - 15 years experience
   - 10,000 hours tai chi chuan practice
   - 10 years teaching experience
   - fewer to be found
   
4. Tai Chi Master
   - 25 years experience
   - 30,000 hours tai chi chuan practice
   - 20 years teaching experience
   - pretty rare
   
5. Tai Chi Grandmaster
   - extremely rare

Peter Southwood has graded Sifu Waller as being a Tai Chi Master based upon his practice quality and experience.

There are 5 levels of Tai Chi Instructor in the UK these days:

1. Health-only Tai Chi Teacher
   - 5 years experience
   - these are the majority
   
2. Tai Chi Chuan Instructor
   - equivalent of 3rd dan black belt in any martial art
   - 5 years experience
   - these people are less common
   
3. Tai Chi Expert
   - 15 years experience
   - 10,000 hours tai chi chuan practice
   - 10 years teaching experience
   - fewer to be found
   
4. Tai Chi Master
   - 25 years experience
   - 30,000 hours tai chi chuan practice
   - 20 years teaching experience
   - pretty rare
   
5. Tai Chi Grandmaster
   - extremely rare

Peter Southwood has graded Sifu Waller as being a Tai Chi Expert based upon his practice quality and experience.

There are 5 levels of Tai Chi Instructor in the UK these days:

1. Health-only Tai Chi Teacher
   - 5 years experience
   - these are the majority
   
2. Tai Chi Chuan Instructor
   - equivalent of 3rd dan black belt in any martial art
   - 5 years experience
   - these people are less common
   
3. Tai Chi Expert
   - 15 years experience
   - 10,000 hours tai chi chuan practice
   - 10 years teaching experience
   - fewer to be found
   
4. Tai Chi Master
   - 25 years experience
   - 30,000 hours tai chi chuan practice
   - 20 years teaching experience
   - pretty rare
   
5. Tai Chi Grandmaster
   - extremely rare

Peter Southwood has graded Sifu Waller as being a Tai Chi Expert based upon his practice quality and experience.

Higher-level Instructors are less common in the UK.

To become a Tai Chi Expert the Instructor needs 15 years tai chi chuan study, 10 years teaching experience and 10,000 hours of tai chi practice.
They must possess extensive, versatile combat skills and more in-depth teaching skills.

There are 5 levels of Tai Chi Instructor in the UK these days:

1. Health-only Tai Chi Teacher
   - 5 years experience
   - these are the majority
   
2. Tai Chi Chuan Instructor
   - equivalent of 3rd dan black belt in any martial art
   - 5 years experience
   - these people are less common
   
3. Tai Chi Expert
   - 15 years experience
   - 10,000 hours tai chi chuan practice
   - 10 years teaching experience
   - fewer to be found
   
4. Tai Chi Master
   - 25 years experience
   - 30,000 hours tai chi chuan practice
   - 20 years teaching experience
   - pretty rare
   
5. Tai Chi Grandmaster
   - extremely rare

Peter Southwood has graded Sifu Waller as being a Tai Chi Expert based upon his practice quality and experience.