The practice of Tai Chi by itself is beneficial to your health in general. The question is: should we conform ourselves with just practicing a little bit of Tai Chi a few days per week? Or by just going to class once or twice per week?
The answer is: it depends on your goals. If your goal is to improve your health, increase mobility, and have a better balance, then a practice of half an hour to an hour a day, three to four times per week, will do this for you.
If you took upon Tai Chi not only to improve physically, but as a martial art, then you need to practice daily. Not only do you need to practice daily, but you need to compliment your Tai Chi with other systems of exercise like Chi Gung (a system of exercises to improve lung capacity, circulation, and the movement and control of Chi), aerobics, weight or resistance training, calistenics, and even Yoga. With a partner, you should learn and practice the martial arts applications of each movement or technique from the Tai Chi forms (empty hand or with weapons). Pushing hands should be part of your training. So we are talking hard work /"Kung Fu" here.
Independent of your goals,
Tai Chi will always give you benefits as long as you keep a serious and consistent practice.
Ush! Chen! Sifu Pedro