Cleaning nadis is one benefit of pranayama, but it is not the primary purpose.
The purpose of the pranayama is to harness the vital energies that are in the physical body and vital body. So, that practice should be done physically, not mentally. You can do it very quietly, you do not have to do it in an aggressive way. You can do a pranayama silently, but be attentive to what you are doing.
Furthermore, it is not necessary to feel physical sensations with pranayama. You might feel something, and you might not. Do not use physical sensations as a measure for success. Your measure should be your state of consciousness. The qualities you observe in your consciousness should be your measure.
If you are doing pranayamas properly and effectively, you will find that it calms your mind, calms your heart, and strengthens your ability to retain mindfulness of yourself. That is the measurement. If it is helping you to remember yourself, to retain presence, here and now, then you are doing it right. If you are doing pranayamas and you are becoming more sexually agitated, you are doing it wrong. You need to manage your mind.
Pranayama takes the steam off the boiling water. The water is boiling because of the fire of lust, and we do the pranayama to harness the wind, to take the energy out, and to let the water settle. That is the purpose of pranayama. It is not to stir up a big fire, it is to take the energy and use it for benefit, so that we are not burned by the fire.
Have you ever seen what happens if there is a fire burning and raging and you take all the air away? What happens? It goes out. Pranayama does that. Pranayama literally means “harness the wind.” With pranayama, you “take the air from the fire,” so the fire becomes calm. And then that energy becomes useful by the consciousness, as it should be. That is how its supposed to work: it should be awakening your consciousness. - From the lecture Transmutation (Alchemy course)
2. Practice as long as you want, but do not bother if you are distracted. It is a waste of effort to be doing pranayama while thinking about something unrelated. So if you can remain attentive to the pranayama, paying attention only to that, for 30-60 minutes, then wonderful. If not, do shorter sessions in which you remain perfectly concentrated on the exercise, then gradually extend the time as you become more skilled at being focused.
3. You can do pranayama anytime. If you are very tired, it can revive you. If you are mentally weak or unstable, it can help you focus. If you are tense or unwell, it can relax and heal you.
4 and 5. There are various foods that play a role, but we are generally so dense and asleep that we do not notice their effects. Each person is different to some degree, so we need to learn how foods affect us. In general: Avoid impure foods. Try to avoid commercially prepared food (restaurants, packaged food, etc). Cook at home. Eat the purest food you can get. Avoid sugar. Etc. It is best to do pranayama when the stomach is not filled.
6. Pranayama should increase relaxation and serenity. Retention of breath can be used, but it should not be extended and make you tense or interfere with the flow of oxygen to the brain. If you are becoming tense or dizzy, stop. You can practice pranayama all day long if you want.
7. The cleaning of channels is only a side benefit. The time it takes depends on how dirty they are.
The purpose of pranayama is to feed your consciousness so you can awaken, eliminate your ego, and serve others.