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.