The answer, like us, is not that simple. At the same time, like yoga, when understood, becomes very simple!
Yoga of the mind is a permanent practice - constant awareness.
Yoga asana (physical practice) will depend on your current needs and will.
One person might not have found the full benefits of yoga and do it once in a while. Others have studied themselves and studied yoga and are ready to personally invest daily.
When should I do my yoga?
Yogis wrote in their texts that the best time to do yoga is in the morning.
The first practice is Saucha - cleanliness. Get your body and mind ready for yoga physical practice and the rest of your day.
Yoga recommends start your practice with no food in your stomach. Though if you feel week or very hungry you should have some rice milk or similar.
Can I do my yoga in the evening?
Of course you can and you should! We are not yogi monks and we have a busy life. The affirmation here is that you should do your yoga practices. No excuse if you cannot do it in the morning. And no excuse that you cannot do it.
There is no point draining our energy and brain power for the benefit of others and not take care - remember is our responsibility - of our own health and happiness.
So what is recommended?
NHS recommends around 3 hours of exercise. We are following this recommendation. We advise for more if you are a yoga teacher training student, if you are reshaping your physical and mental bodies. The more you do, the more you help yourself or help preventing stuff that is yet to come.