For most people, practicing yoga three times a week is ideal. If it is hard to get to class that often, than I suggest trying to do some kind of similar home practice, so you can maintain the progress that you make in class.
Particularly when working on flexibility, I find that consistent practice and/or stretching is more useful than intense but less frequent work. For many of us, the most difficult part of class is the mental hurdle at the very beginning, when we are confronted with how tight our hamstrings or shoulders feel. My experience is that the more consistent the practice, the more quickly one moves through the creaky period at the beginning. This is true mentally as well as physically. When we are in a consistent rhythm with our practice, we are more likely to understand its ebb and flow. Some days we feel stronger and more flexible, other days feel like a struggle. The main thing is not to judge or constantly try to chart our progress.
So make a habit of supplementing a weekly or twice-weekly class with some gentle work at home, and you will probably experience the physical and mental benefits of yoga on a more consistent, ongoing basis.