The active exercises all work to clear channels, remove blockages , etc. One of them done correctly, like the moving of yin and yang, for instance, can do this. Your intentions and visualizations help this happen, as also does your singular focus within 'an' exercise. It doesn't take two hrs to clear yourself. So, one 'could' look at the purpose of the extended time in an active exercise.
Time in the emptiness is the goal. If a singular position, or exercise gets you there, this is just as valid as going through sets and sets of exercises. The amount or number of exercises don't gaurantee the success. More, how well you focus the mind, or reach no mind, or emptiness. The body heals itself in that state and you further a transformational process of self realization in this practice of mindfulness.
Two hours of active exercise in one session is alot. If I was new to qigong(1-2yrs experience), I would try a more balanced approach. Doing an hour session is a great initial goal. Doing a variety is good, both within active, and also incorporating meditation. One should spend time , initially, learning what it is, what the exercises are doing, what works for you, what engages you.
An hour of active, and an hour of meditation, positions you to experience the widest possibilies within this practice. After a time, it is possible for 'you' to make very clear decisions about your own practice. This is a self empowering process of gaining insight and wisdom too, that comes from within.
One source of wisdom would say, "avoid extremes". Two hrs might be too extreme for one person, and not another. You want to integrate qigong into your lifestyle. If you start out too strong, you might drop it, or the time requirement might not go forward well. Building a balanced gradual practice, and being consistent is a good way to go.
Regarding the seasons. Jetay is right about the time. It relates to the time it takes for qi to make a complete cycle within the body. Four cycles, of close to 30 minutes each, is a year, or four seasons. It doesn't have to happen back to back. It can...
Again, if I was fairly new to qigong, I would break up the sessions, shooting for the minimum of one cycle( @28/29/30min), and the preference of two. Some time is lost getting into an exercise, so two cycles gives you better results,,,,in general. I would do half active and half meditation, leaning a bit towards active if I was ill or had a physical blockage I was clearing. This is not true for everyone. Meditation certainly holds the means to do it all. Most people seem to have an easier time with the 'hands on' so to speak, of moving the body and visualizing, with meditation being more a new concept for them to get used to.
At a very advanced level, spending two hrs in active can be very, very good. I speak of emptiness again. If I was going to do active for two hrs, I would pick one exercise. If it moved, I would slow it down to where someone watching me wouldn't easily see it move, or not see it move. I am talking slow. Slow the breathing too. The amount of reps doesn't add anything. One rep is great if you can do it! I feel this is quite advanced, but would yield great results.
Two hrs sounds good, but will you be able to establish or maintain a balanced practice? Do you know yet what that is, relative to you? Will you be able to integrate qigong into your lifestyle for the long haul?
Would you ask us what to have for dinner tonight? If not, why. Maybe because you know what you want to eat based on experience.