Yoga is interesting to practice, even if you practice one hour in a week, you will get the advantages of the practice. Practicing more than 1hr/week will definitely earn you more benefits. I am of the opinion that starting with two or three times in 7 days, for just 1hr or 1hr30mins per session will do. If your ability can only take you for 2o minutes per session, it’s equally okay. Don’t allow time factor or irrelevant activities disturb you—do your best and be happy about it. You are likely to later find out that your zeal to practice will increase naturally and you will find yourself practicing more frequently.