Should one practice vipassana every day in a formal sitting? If so, how long should it be compared to samatha meditation, should it be practiced right before/after (not asking which comes first!) samatha or at anytime during the day?
When meditating from person to person and even from session to session the following can happen:
- insight develops before calmness
- calmness before insight
- neither calm nor insight
- both calm and insight
Calmness and insight should be balanced through personal effect and also you should seek guidance on balancing both. [(Samatha Vipassanā) Samādhi Sutta 1 - 3]
Also it is helpful if the calmness is developed through concentrating on one of the Satipatthana [Saṅkhitta Dhamma Sutta] as the object of meditation is not tainted by a Vipallasa [for more on Vipallasa see: Vipallasa Sutta].
I would like to recommend a great book: Mindfulness in Plain English by Ven. Henepola Gunaratana:
I learned that it is good to practice various types of meditation until you are quite comfortable with them.
After that, a normal meditation session could start with Loving Kindness meditation, followed by Samadhi until you get a good concentration, then continue with Vipassana (to see the 3 marks of existence).
Enjoy your meditation.
If you can practice any form of meditation on a daily basis, that's wonderful! The key is to let it happen naturally, and not try to do more than you are ready for. Keep meditating, but also be patient with your progress.
Regarding the ratio of time spent in shamata vs vipassana: it depends on your current ability. You will find that you naturally spend more time in vipassana as you gain experience in both types of meditation. That said, some days you need (want) more shamata, sometimes you want more vipassana. It can change day to day.
By the way, it's okay (and even recommended) to mix shamata with vipassana during a given session.
Basically, do what feels right. Don't over analyze it. Just do it.
In my Buddhist lineage, it is generally taught that shamata supports vipassana, and that has been confirmed by my experiences (many years of practice). Stable, powerful insight depends on having a strong foundation.
You don't need Samatha to practice Vipassana. Vipassana is the one and only path to enlightment. End the suffering once and for all.
The more you do Vipassana, more you'll be closer to Nirwana. Saying that, don't try too hard. Don't try to grab Nirwana forcefully. Give up Nirwana from your mind too. Vipassana is trying while not trying.
Should one practice vipassana every day in a formal sitting?
No, you should practice Vipassana in all of your postures. Formally, there are four meditation postures (sitting/walking/standing/lying).
You can read more about postures here: Postures for Meditation.
If so, how long should it be compared to samatha meditation, should it be practiced right before/after (not asking which comes first!) samatha or at anytime during the day?
Samatha and Vipassana are totally different.
You might want to watch this: Ask A Monk: Samatha and Vipassana.
See also: Vipassana Instructions.