Is there any indication given in the Alien movies or literature of how long an Xenomorph lives for? It occurred to me that given a limited number of potential hosts ( as per the colony on LV-426, the number of suitable gestation vessels would soon be depleted as would any other sources of food.
I was watching a video on Alien and they said that the Xenomorph wasn't hunting Ripley when it was in the escape pod, rather it was looking for a quiet place to die. They said the color change (light to dark) was showing it's lifespan, the older it got the darker it got. I have yet to fact check this but I found it on youtube. The name of it is 25 things you didn't know about Alien (read the exact claim in this page), also see the video, you can find the exact spot at 12 minutes and 31 seconds into the video. Youtube wasn't working so I had to use a different site.