The first yoga books I used to teach myself prescribed 2-3 repetitions for each pose. If I followed that suggestion, it would take at least 60 to 90 minutes to complete my daily regime. While such a regime would benefit me, I would not be able to carve out that much time daily for yoga practice. What is important is that I put everything I got into the stretch or hold. When I do, I feel I can benefit from doing the pose once as much or almost as much as repeating twice or three times.
In general, let your personal fitness and body needs as well as time constraints dictate how many repetitions you choose to do of a particular hatha yoga pose. Most days, I do a pose once. Other days, I repeat certain poses twice or three times if my body guides me to do that. For example, one afternoon I was not paying attention and I sat in an awkward position at my office desk for hours. Boy did my lower back become sore and stiff! Luckily, I remembered that hatha yoga could resolve the situation! The next morning I repeated the cobra and the bow twice each to work out the kinks. Voila - problem fixed! Some days, though, I choose to repeat certain poses just for the joy of it! Everyone has different bodies, issues, and preferences.
Concerning the time you hold the poses, I have been surprised that many books neglect to tell you! However, I hold most poses for the time recommended by my first yoga books. Of course, when you begin, you will be holding the poses for a few seconds maximum. Thus, initially your daily practice may take only 10 minutes or so. That's ideal. You want to adopt the yoga habit gradually. Starting out with 5 or 10 minutes a day is an excellent strategy for creating the discipline of a daily practice. Experimenting and working your way up gradually is the key to long-term success with your at home yoga practice.
Wisdom of the Day: "Put everything you've got into a hatha yoga pose." Laura V. Rodriguez