Each of our body condition is different. It will be good to approach a qualified yoga teacher for some lessons before we start practicing on our own.
Duration of the practice will depend on how much time we can spare. Some of us are fully occupied with work and house chores, daily practice of 1 hour will be impossible. Set a duration that can be maintained and committed daily. Short duration like 10-15 mins will be fine as well, so long we keep doing it everyday. We will be able to feel and see results.
I prefer to do it at the start of the day, it gives me more energy that I need, and refresh my mind.
For you ?