That's a good question. Deciding how much you practice really depends on what your goals are for yourself. Are you in maintenance mode or are you in healing mode?
Maintenance Mode: If you are using Bikram Yoga to maintain your current good health, to create more strength and flexibility, to help control stress, as a moving meditation and space for your personal growth, then practicing 2-3 classes per week is enough to achieve these goals.
Healing Mode: If you want to really change your body, lose weight, heal an injured body, lower blood pressure, normalize blood sugars related to type 2 diabetes, or a myriad of other health related issues, then the more you practice the quicker you will heal your body. 4 to 5 classes per week is a good way to set yourself on the road to a healthy life style.
Don't worry, you won't be in healing mode forever. Once you achieve your goals, a maintenance practice (2-3 classes per week) will help you sustain your health and wellbeing.
This is a healing yoga and a steady, even daily practice, just doing what you can in class, will help give the body what it needs to heal. The human body is incredibly self healing when given the right tools. The increased circulation, the heat, breathing, gentle stretching, movement of the lymphatic system all aid in the healing process. Even if a student can do very little in the room, there is great benefit to coming in and working your range of motion. Students can practice in a chair or on the floor, for some people just sleeping for 90 minutes in the heated room can provide great benefits. Spending time focused on yourself and letting go of everything outside the room for ninety minutes can be extremely healthy.
This is a healing yoga. Students who have an acute injury, a recently herniated disc, recovering from surgery, muscle injury etc, heal much faster by practicing more. Many students take time away from their practice when they have an issue, but this is the time to up your practice. It breaks my heart to see a student stay away during the time they should be in the room. When they return, their acute issue has often become a chronic problem which requires a longer period of time to heal.
Consistency is always the key. We all have busy lives and getting to class sometimes is hard. Making yourself a priority is important but if you are just maintaining your health and working towards a stronger more flexible body, then even if you are only able to come one day a week, if you come consistently once a week, you will see changes in your health, your practice, and your life outside the room.