I hear this question alot. In general my opinion and my answer is "as often as possible, but a little is better than none". It's like any other form of exercise or sport...the more you do it, the better you get or the more progress you will see. But for some reason people want to hear a number, a target, or some sort of formula that will lead to yoga bliss. If genie's granted wishes...
Bare minimum: 1 day per week. Once a week is a good way to offset or cross train for whatever else you do for fitness or sports. It's a good way to unlock those hamstrings and pecs from running, biking, lifting weights, etc. Coming once a week is a good place to start. Although I am a firm believer in a little bit of yoga is better than none, as an instructor I am always concerned about safety during practice and avoiding injury. Someone told me a long time ago that it is easier to just be an “A” student than a “C” student. The “C” student has to work harder because they slack off for a period of time, then torture themselves playing catch up and cramming versus the student that just does their work all semester long and doesn’t need to pull all-nighters because they’ve followed the syllabus.
Twice per week. You like yoga a lot and you feel really good after class, but can barely squeeze in time to get yourself there. I get it. I really do. Start coming twice a week for 6 weeks and feel a huge difference in your body. You will see an improvement in range of motion, minor aches and pains go away, your strength will increase, and you might even shed a pound or two.
Three days per week. You did the twice a week thing for a while and you are really hooked because you noticed improvement in strength, flexibility, mental clarity, quality of sleep, and you even lost a couple pounds. Three days a week will be better. As far as yoga goes, I am a believer that more is better. After over a decade of consistent practice, I can say with confidence that I have gone through phases (like anyone else) of practice. When I practice 3 times a week, I feel awesome, but I still feel like I'm struggling in class.
Four to six days per week. You come to class three or four times a week, and practice at home with a video or something on the off days. Talk about feeling awesome. This is when you will see your body, mind, and practice just explode into goodness. It's a major commitment, but the reward is health and feeling great - naturally.
Practice, practice, practice. You will get better, and the minor aches and pains from starting a new fitness regime will subside and you will feel great! Remember: practice, practice, practice...and drink lots of water after class.