Hi,
I like the first answer, as to what would be optimal, being 'what works best' for you. I am certainly not the bottom line reference regarding your question, although I would like to offer some additional considerations related to your initial post.

- Consider the experiential aspect of the practice versus the academic. I would say another perspective at considering what the optimum practice is, for you, is at some point, to immerse in all aspects of the practice, to find out for yourself what those aspects are. Experiential has a relationship to developing the intuitive. The more time you spend in any one aspect of level I, the more experience you have with it, and the more you know what it is relative to you, and what's best for you. It would not be so good to approach your practice like getting on a treadmill and setting a timer.

- My sense of say small universe versus self concentration, is that two distinctly different approaches to meditation are given.
Self concentration seems to me , a bit like the Taoist Inner Smile meditation , in that the initial alignments given embody happiness. Also, it has an aspect of informal, and letting go. Small Universe is very different, in its more specific structure and moving focus. In the long run, Small Universe might be something that is more likely to remain in your qigong toolkit, but I think it is best to stay open to every aspect included in level I, and spend time with each element, and be flexible on your feet. You change, the energy of each day is different. You want your practice to be consistent, and you also don't want to intellectually lock in some perception of what it should be. There is a rigidity in that. You can be led by your practice experience.

- It is optimum, to integrate qigong into a lifestyle practice. I don't know what the drop out rate is, but I would assume that not all people that come to qigong, stick with it. I sometimes look at the 28 minute cycle of qi relative to how long a long form of Tai Chi takes. There is a similar time frame. Doing say, a Tai Chi form everyday for many years, is great. Same for 30 minutes of qigong. If you managed to integrate qigong into the rest of your life for 30 minutes a day, that is better than a plunge into 3 or 4 hr qigong practice for a few years, then stopping.

- It is easy to self judge oneself, relative to how one see's self doing this practice. It is not constructive to implant the notion that something is optimum, and then on some level 'know' that you are doing less than optimum, when you can't accomplish what you set up as optimum. It is also harder to respond to change, if you lock in a definition of what you think you need everyday.

-The optimum, is emptiness. This would speak to quality versus quantity. If I had a choice, I would choose ten minutes of genuine emptiness over one hour on the qigong treadmill. A deeper stance relative to active, or a lotus sitting position relative to meditation, changes the quality of time spent. One hour meditation in a lotus position, is different than one hour meditation lying down.

- There are 24 hrs in a day. Which is optimum, being happy for all your waking hours, or doing a qigong exercise for two hours.
Focus on a qigong exercise for two hours, or bringing focus into your workday. This relates back to lifestyle practice, but I bring it up again to consider important elements that are outside the physical practice, but relative to personal transformation. Qigong supports, helps, or can lead one into broader lifestyle awareness's/perspectives. The physical practice is one thing, the spiritual/mindfulness another.
Many people tend to think that standing a certain way and moving
your arms, is the essence of the qigong. Keeping that smile embodied, or happiness, or holding a focus, are such important elements in the practice, and those elements can extend beyond the confines of a practice session. Two hours of smiling inward, is better than two hours of moving your arms and dropping the smile. You could say, just relative to this, that being happy is optimum, and also, developing a strong focus. Developing focus, starts with minutes or even seconds. Again, it is good to consider these things when you look at your optimum practice, and what the quality of that practice is. Too much focus on time spent can miss the point.

- My impression of this two hour mark, is that it is applicable in one session or as an overall template for the day regarding exercise/meditation sessions. Good, better ,best relative to all at once, or in parts. If you want to teach, do more than that. If you have a serious illness, try to do more if possible. If you have a life/purpose/purposeful career, utilize qigong to support it, not supplant it. Finding the connection between practice session and lifestyle is very good. Focus at work, be happy in life.
love,
gallen