Once a Facehugger leaves its egg, how much time does the organism have to find a host before it dies (or possibly has to reenter the egg)?
Given the nature of the creature as an ambush predator (the delivery system is activated only when suitable prey comes into range) it is possible the creatures might not need to live for longer than a few hours. Either it is strong enough to impregnate its potential host or its not.
- Given the creature's formidable strength, amazing speed for its size, formidable defense system (acid for blood) it wouldn't need to live long, just long enough to plant the gene-package which becomes the new Xenomorph. We have seen them in storage tanks awaiting implantation or being held alive for later dissection, (in Aliens II, the slimy corporate has a jump-scare with one in a tank) We know they can live for at least a short time pre-implantation.
The Xenomorph exhibits an r-selection reproductive strategy. When the queen drops eggs, she drops hundreds at a time. A standard insect process when the r-selection strategy is to produce as many eggs and offspring as possible, and who are born with all the instincts necessary for survival. It is also a strategy used when the environment may be unknown or hostile, much like the ones Xenomorphs tend to set up their egg groups in. This is the opposite of the k-strategy which produces a low number of highly dependent offspring whose intellectual capacity is far in excess of most r-strategy lifeforms.
Given their alarming success rate for successful implantation (accounting for Ripley and one Predator I've seen escape implantation) they have a 99.5% success rate of implantation in most prey. So dying if they miss in a few hours seems like an acceptable loss ratio if they are unsuccessful.
As an added thought, given the longevity of the egg form, they appear to be able to sit fallow for decades, perhaps even centuries, nature isn't usually so generous with her gifts. Long shelf life probably indicates a short active life since the nymph creature would be geared for resting longevity, not long-term prey tracking and hunting.
Another clue may be the creature doesn't appear to have any form of sense organs. Beyond the ambush attack, the creature doesn't appear to have or need any long-term development of external sense organs. Granted, insects don't have external ears, they usually have clearly indicated eyes which I have never seen on any of the nymph stage xenomorphs.
Here are some of the many clips of facehuggers doing what they do best. Trying to implant their prey. It is probably best listened to with the sound off.
That is a very good question, for which we can only guess the answer:
It takes up to 24 hours (first Alien movie) for the facehugger to impregnate their host, during that time they do not seems to feed, so their life span should be pretty short
HOW LONG DO THEY LIVE? The average xenomorph life span is currently unknown, at least as far as the films go. While the eggs aboard the derelict vessel had remained there undisturbed for thousands of years, they may have been preserved via advanced technology. Ridley Scott thought the creatures led very short lives -- so short in fact that the creature is actually dying during its final encounter with Lt. Ripley.
The creature in first movie is found dead shortly after impregnating Kane but its unsure has it died because it finished the job or simply from the "old age" - my guess would be that the impregnation would shorten the lifespan, because the process requires quite a lot of energy from the hugger. Biological organism usually don't have the "Self destruct" buttons, creatures like mayflies die from lack of energy, not because they impregnated female/laid eggs.
But then the facehugger doesn't have to live long - it its supposed to simply hatch, jump at the immobilized victim, impregnate and die - besides his powerful jumps he is rather clumsy and definitely not suited to "hunt for the host" for a longer period of time (in the AvP II game such hunt happens at the beginning of the Alien campaign, but its quite short). So this would suggest that its life can be measured in hours.
On the other hand, in "Aliens" we can see live facehuggers floating in some preservative liquid for a quite long period - longer than Sulaco's journey (I'm not sure how long exactly, but the ship's journey was long enough that required crew's hibernation). One could argue that either the liquid is somehow feeding them or that they are stronger (and therefore living longer) because they have been hatched from newly laid eggs (contrary to one from "Alien" that was born from ancient one). This information would suggest that facehuggers can live for quite a while, with lifespan measured in days or even months, although we know that Aliens can enter the hibernation if their activity is not necessary:
The Alien’s apparently impending demise was telegraphed through its lethargy aboard the Narcissus shuttle, in addition to a (hardly apparent) disintegrating paint job on the creature suit. In Aliens, the creatures at Hadley’s Hope are weeks old, scuppering the idea of a severely limited lifespan – with this in mind, we can easily chalk up the original Alien’s lethargy to it entering hibernation, much like the Aliens within the Atmosphere Processor before they are disturbed by the Colonial Marines.
So to sum up my answer: facehuggers in normal conditions can barely survives short time (hours at best) without finding a proper host.; the impregnation process takes huge amounts of energy from their bodies which results in their death shortly after the act. However as the adult Alien they are able to fall (in optimal conditions) into hibernation, that can extend their lifespan significantly (facehuggers in tank in Hadley's Hope), akin to hibernating insects (the whole life cycle of xenomorphs is influenced by parasitic wasps or phorid fly, whose larva eats an ant's brain).