It depends on your personal situation and your goal of practice.
In Tai Chi history, many masters practiced at least 10 times of routine everyday.
But it is not easy for people who are living in modern fast-pace society to practice like that. However, there is a way of maintaining your practice and even make some daily progress even though you do not have a lot of time: you should practice the basic techniques such as silk-reeling exercises, some posture-stances, and single movement exercises. By this way, you will build and maintain your basic foundation. Then practice routine once.
In the Tai Chi community of China, people say that:
You will keep your level if you practice one routine a day;
You will make a bit progress if you practice twice routine a day;
You will make more progress if you practice three times routine a day.
However, if you want to be professional, you have to practice as much as you can without counting the number and hours every day. Eventually, your practice and demonstration will tell others how many times you have practiced.
So set up the goal first then find the best time and practice it, you will get benefits after that.
Thank you for the feedback