How to balance samatha and vipassana practice? 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].