I am not sure if I understand your question, but agree with what Pete said.
You could also say that doing Tai Chi forms is like reading a story. You can get some enjoyment and some benefit from doing just a little bit; however, it is more rewarding to do the whole thing.
As for the length of practice, most of the clinical evidence nowadays supports exercising for at least 15-20 minutes per session to maximize the benefit you get per minute exercised.
If your concern is learning such a large number of movements, I would say: "Be patient." If you make Tai Chi a lifetime activity, 108 movements is not that much, even if it takes a year or two to learn in the case of some people. If the number of movements is still a problem, there are forms that have as few as 13 moves or fewer, but that will usually not give you the same benefit.