It's better to do it with quality than to do it without quality. If you can only do it once every other or every second day it's better to do skip a day than to practice without quality. T'ai Ch'i requires a lot of the left brain, meaning you have to memorize a lot of the steps. That means it requires a lot of time before you get the benefits. You lead into more of the relaxation of the movements and focus on the quality of the breath.
Different people take different times.
For many people they experience some benefits after only a week. For those who are out of shape it will help get you in shape. For those who are very athletic it will help finely tune your body. If there is any handicap or any disability or motion limitations you can modify any exercise to fit your own mobility or capability. Meditation postures that are shown on the video can be revised to a sitting position or lying down position.