How much citric acid to preserve my juice
Short answer: Enough citric acid to get the pH below 4.2, then pasteurize. One mM citric acid solution has a pH of 3.2, but it's essentially tribasic with pKa of 5.21, 4.28 and 2.92, so figuring it out on paper will take some work. It might be easier to just add some acid and see what the pH is, repeat as necessary.

References: Tetra Pak White Paper on Pasteurization, Citric Acid wikipedia

How much additional storage time will you get? I cannot answer that, there are multiple other ways foods can spoil, storage conditions are not mentioned (temperature, storage vessel, etc.), and generally without lots of careful work one cannot achieve at home the storage longevity of commercially-produced items.