At least 30 minutes for Yang Style 108.
At least 15 minutes for Chen Style Old Frame First Routine
At least 12 minutes for Sun Style.
Why? Because when you practise, it is important to practise slowly. Because when we first learn taiji, we still don't know how to relax. So when we move too fast, most of the time, we are not able to move in a relaxed manner. Moving slowly forces us to try to relax and gives us time to focus on the fundamentals, such as making sure the movement of the hand is linked to the legs.
Once you have reached a certain stage, you will be able to move in a relaxed manner fast or slow. It is still recommended to practise slowly though, so that you can constantly check your actions to make sure they are correct.