The short answer is between one and six times a week. Once a week will keep you in touch with yoga but progress will be difficult.
Yoga can be tiring initially. When you start practicing there is so much new information that one can quickly become overwhelmed. Many people find that they can only practice two or three times a week at the start and have to build up gradually.
The most you would ever want to do is six times a week. It is important to have at least one day a week of rest. Otherwise you will become exhausted and not be able to continue.
If you are just beginning yoga or thinking about trying it out, you may want to try a combination of once a week in class and perhaps two or three times at home.