How long can a Xenomorph live without eating anything?

Well, canon wise Xenos can hibernate for a very long time.

The original idea however was that they only live a couple of days.

As long as they need to in order to serve the plot and storyline. :D

I myself stick to the lifespan of being a couple days at most, the adult form doesn't eat, that's the chestburster-stage's job.

No one has lived long enough to find out and the xenos have all died relatively quick. Again, aliens, movie, plot devices, antagonists, people die, move on. I honestly don't care. Too much fan fiction stuff when none of it matters. Can't wait for the next movie. Hopefully it won't spend any time on them eating unless it's humans:) no offense.