Learning tai chi chuan can often be quite challenging. It takes time to retrain the body to move in the correct way, learn new skills and be able to apply these in combat.
Make good use of your time between lessons and train at home. This is where you explore what you have learned in class.
If you find that you do not have much time because of other commitments then training little and often is better than not training at all.
How long would it take to gain significant skill if the only time you trained was in class?