Yoga is amazing—even if you only practice for one hour a week, you will experience the benefits of the practice. If you can do more than that, you will certainly experience more benefits. I suggest starting with two or three times a week, for an hour or an hour and a half each time. If you can only do 20 minutes per session, that's fine too. Don't let time constraints or unrealistic goals be an obstacle—do what you can and don't worry about it. You will likely find that after a while your desire to practice expands naturally and you will find yourself doing more and more.