Historically a Master Craftsman or Master Tradesman would proceed through a 3-stage career path:

1. Apprentice
2. Journeyman
3. Master

This intensive approach to learning is tried and tested.
It has been the source of inspiration in modern times for changes in the UK education system.
Sometimes the old ways are better.
There are lessons in the past.

Although it might seem that the necessary time to master the requisite skills and attain a level of expertise would depend upon the field and your own talent level, those who have researched the subject repeatedly come up with the number of 10,000 hours. This seems to be the right amount of quality practice time that is needed for someone to reach a high level of skill and it applies to composers, chess players, writers, and athletes, among others.

Although the number of hours seems high, it generally adds up to seven to ten years of sustained, sold practice - roughly the period of a traditional apprenticeship.
(Robert Greene) 

Most tai chi students are content to attend classes, enjoy themselves and make leisurely progress through the syllabus.
They are not overly ambitious.
Some students are different.
They seek to acquire the inner teachings, become an indoor student, a lineage disciple and truly steal their Master's art.
These students are essentially undertaking a tai chi apprenticeship; a lengthy journey of study and practice.

After some years of practice your Master may ask you if you potentially want to become an Instructor.
The training will commence with helping out in class.
Simple chores and assistance.

If you show enthusiasm and competence, your Master may commence teacher training with you.
This formal process of learning will take many months, probably even years.
Once qualified, an Instructor should seek Instructor affiliation and insurance with The Tai Chi Union for Great Britain or equivalent body.

To become a Master, the journeyman needs to have put in the hours and have acquired a level of understanding and competence concordant with mastery.
This entails an immense personal sacrifice in terms of time and effort.
Serving the art may sometimes feel like a thankless task; seldom offering financial reward, frequently being second-guessed and taken for granted.
Master-level tai chi chuan is not a matter of new forms and material.
It is to be found in the comprehensive nature of the practice. 
The thoroughness of the understanding. The simplicity, sensitivity, softness and ease of ability. 
Every action should contain the tai chi chuan principles.
All of the exercises, forms and drills are dismantled so that the exponent can examine how and why they operate.
This is a crucial stage of learning; enabling the individual to truly comprehend the art.
Mastery is versatile and complex - demonstrating a broad degree of insight.
It is also remarkably understated.
The exponent becomes lost in the art; yielding to its intricacies, methods, ways and patterns rather than seeking to dominate it.

In-depth concerns must be explored thoroughly: biomechanics, consciousness, consolidation, Eastern philosophy, form, kinaesthetic awareness, martial theory & practice, meditation, proprioception, syllabus design, spiritual inquiry, tai chi principles and teaching methods.

The journeyman phase of practice and study will involve a further 20,000 hours of practice.
The equivalent of another 14 years full-time study. 

Most Instructors stay at this level because the logistics of work, family and making a living make it very hard to commit the necessary amount of time to tai chi.
Reasonably, an exponent must start young if they wish to truly Master tai chi.

Sifu Waller overcame this obstacle by quitting his day job and studying full-time for 10 years, practicing tai chi and bagua for countless hours, reading, reflecting, writing about tai chi and teaching classes throughout the week.

The student needs time to digest what they have learned, to consider new avenues and directions.
Although the apprenticeship phase is essentially over, the Master/disciple relationship continues. 
There is still much to learn from the Master.
The Master now watches from a slight distance; allowing the student to find their feet and discover their own proclivities and preferences.
The journeyman should be the equivalent of (or higher than) a 3rd dan black belt in any martial art.
They will understand how to practice the art and be capable of teaching other students.

An Expert will typically open their own class or school.
They may continue to teach on behalf of their Master.
It depends on the individual.
Being a Tai Chi Expert is more than just talent.
You need to put in the work.
The journeyman should aim to make a great leap forward in terms of tai chi skill. 
They must begin to see connections and associations between different aspects of the syllabus.
Taoism, the Tai Chi Classics, martial considerations, biomechanics and teaching blend together in a new kind of understanding that transcends verbal explanation.

The initial apprenticeship stage lasted for the equivalent of 7 years: approximately 10,000 hours of quality tuition and practice.

For the tai chi student this initial phase of learning will furnish the exponent with all the necessary skills for the journey ahead.
Intensive teaching, corrections, study, repetition and refinement are necessary.
Patience is required.

Only the Master can determine when the student has completed this first stage of learning.
At a certain point it will be time for the apprentice to spread their wings and explore the art further for themselves.