In traditional Tai Chi training, which we view Tai Chi Quan as a Chinese Internal Martial Art, I believe the main purpose of Tai Chi form training is to develop Jin (劲) or internal power. Thus, the measurement of the training success should be if or how much Jin have you got through the form (or other solo) training.
For example, in traditional Yang Style Tai Chi, the number one Jin is Peng Jin. Here Peng Jin is different to “Ward Off” movement. Peng Jin should be part of any movement. If you haven’t got Peng Jin, then your form is still not right regardless how good it looks like or how many medals you have won. Once you obtain Peng Jin, the more you practice, the more internal power you will develop. That will also lead to other Jin development such as Lu, Ji and An.
Unfortunately, in the Tai Chi form competition circuit, judges who can see the internal martial ability are rare. Thus, lot of time, the competition result is not a true reflection of one’s real Tai Chi ability.
Of course, there are many points to be trained in your Jin development such as Song (松), body posture, correct movement etc. The details are not in the context of this article.
Besides, form training should be for quality not quantity. Practice a lot of forms is not going to help you develop your Jin. Put more effort into the movements within the form is more important than doing many forms.
Remember Tai Chi training is for your own development not for performance for others to see.