Years ago(actually more like decades ago....) I taught myself yoga by reading Yoga for Beauty and Health and other classics that I still refer to frequently for pointers and reminders. I particularly love Yoga for Beauty and Health because it provides a clear, easy-to-follow, step-by-step explanation of each pose and its benefits. It also gives you precise instructions on how long (seconds or minutes) to hold each pose. I have found that contemporary yoga books often neglect to tell you this! Nothing like leaving you in limbo.
In contrast, Yoga for Beauty and Health advises that a beginner doing the alternate leg pull hold the pose motionless for 10 seconds the first week and then every subsequent week add 5 seconds until the beginner has worked his or her way up to 45 seconds. For other poses the holding time can last even longer. The beginner is advised to repeat each pose two or three times. If you actually do this and follow the recommended practice sequences and sets, you will likely be on your mat 60 or even 90 minutes or more!
While it would be wonderful to take such time daily, you would probably balk at investing that much time. It's just not feasible for many people, especially if they have a family and schlep to an outside office daily. Over the years I have modified the number of repetitions for each pose and the time period for holding each pose based on my intuition. I repeat maybe only one or two poses - the ones that target my "special needs" zones. Also, I rarely hold any pose for more than 20-25 seconds each. I seem to gain tremendous benefits from this regime! If I can fully enter and hold a pose comfortably, I have concluded that 20-25 seconds per pose are giving me the benefits I need.
Yoga is about you and become attuned to your body, mind, and spirit. It's about unleashing the splendor within. In a home practice you have the freedom to choose how to achieve that. And, 25-30 minutes daily has enabled me to stay slim, supple, and serene (the serene part -most of the time!) and feeling fabulous.
Yoga Quotation of the Day: The objectives of Hatha Yoga are twofold: 1) to cultivate the natural beauty of the body and attain a high state of health; 2) to awaken a great power that lies dorman and use it to develop one's unique individual potential - i.e., to achieve self-realization.~ Richard Hittleman in Yoga 28 Day Exercise Plan.