Since the Aliens are not released until the Predators obtain the plasma cannons , they would have had to wait for at least day before they could hunt the creatures, had the gestation time been the same as in the previous Alien films. It would be logical for the Predators to speed up the life cycle. The manner in which they accomplished this is uncertain.
One possibility is that the Aliens' life cycle becomes faster in the atmosphere they're exposed to. In other words, on planets with lesser atmosphere, their life cycle takes longer and on planets with stronger atmopshere, their life cycle becomes faster.
*Lets recap*
Alien: On LV-426, there was probably little oxygen (it was a 'primordial atmosphere' according to Ash). It took about 24 hours for an Alien embryo to grow and almost 24 hours for an Alien to mature. And the rest of the story continued aboard the Nostromo, which has oxygen tanks but is still different from Earth's.
Aliens: Also on LV-426, this time with a breathable atmosphere. However, it is probably not ready yet (lower oxygen content than on Earth). We see one person being attacked by a Facehugger (in the Special Edition), but we don't see the Chestburster hatch. We also have one woman being killed by an Alien bursting out of her, but no info as to when she was implanted. The fact that one subject was killed when the Facehugger was removed before implanting suggests this process takes considerably longer than a few minutes.
Alien3: Took place on Fury161 which had a different air than Earth, which would explain why the Alien grew faster than in Alien but slower than in AvP. The embryo in Ripley took about a week to grow because the Alien was a Queen, which takes longer then the other Aliens.
Alien Resurrection: As in Alien, the movie took place on a ship, which has oxygen tanks but the air is different than on Earth, which would explain why it took a few hours for the Alien at the end to break out.
Alien vs. Predator: Took place on Earth with stronger atmosphere; on Earth, it may take 10 1/2 minutes for the Aliens to grow, much faster then on LV-426, Fury161 and space ships, which would explain why the Predators picked Earth, because it also had humans they could use. And the PredAlien inside Scar Predator took longer because it was a Queen.
Aliens vs. Predator Requiem: Facehuggers on the Predator shuttle were in spaceship oxygen for who knows how long, and as in Alien and Resurrection, it takes longer when exposed to air tanks, which is why it took a few hours for the Facehuggers' embryos to mature and hatch. And the rest of the Aliens were directly implanted into women's bellies by the Hybrid. In any case, they were implanted on Earth, which is why it took about 10 1/2 minutes for them to grow up.
(Theory 2) It's possible that the Aliens mature faster when they've recently been born. In other words, the younger the eggs and Facehuggers, the faster the Aliens grow and mature; the older the eggs and Facehuggers, the longer it takes for them to mature.
*Lets recap*
Alien: The eggs were perhaps thousands of years old, which could explain why it took 24 hours for the embryo to be implanted and mature, and almost 24 hours for the Alien itself to grow to full height.
Aliens: Same eggs as in ]Alien, which goes a long way to explain why these Aliens take some time to grow and mature. Also, the Queen was born from this hive, so she could also lay 'old' eggs with a slowed-down cycle.
Alien3: One egg originating from a Queen that was born from an old egg. The other Facehugger in this film was a Queen Facehugger. It took a few hours for the Dog/Ox Alien (Depending on which version you watch) to be born. As for the Queen embyro, it may takes a longer time for a Queen to grow.
Alien Resurrection: The military might have had eggs for a while and seemed to have bred 14 Aliens (including the Alien that blew up in the escape pod and the first Alien Ripley killed) in a few hours or days. So the eggs may have been laid a few days before implantation. There were seven people implanted and there seemed to be about 15 hosts, which means they may have bred seven Aliens at a time. Purvis was among the first batch of people to get implanted (he can be seen among them in the implantation scene), so it is unexplained why it takes his Alien until the end of the film to out of his chest. Perhaps he is in ill health, which may slow down the growth.
Alien vs. Predator: The team in the sacrificial chamber was implanted a few minutes after the eggs were laid, so the eggs were fresh. The Queen was older, but the Predators' methods to keep her in cryostasis may preserve her better than the ship on LV-426 did. That may explain why they matured so fast. The PredAlien at the end is a Predator/Queen hybrid, which explains its longer maturation time (a few hours, compared to several days for the Queen in Alien 3).
Aliens vs. Predator Requiem: The Facehuggers in this film were kept in tubes for who knows how long, and after they found a host, it took about 24 hours for the Aliens to grow. The embryos the Hybrid laid in the women's bellies birthed a few hours later because they were just laid.
(Theory 3) My opinion is that all along, the Aliens have been hinted at as being engineered. If it was a weapon of war, you want some time from implantation to expulsion, so that the host can get back to his own kind and spread the infection, and to make sure the Chestburster doesn't pop out before the host is back where you want him or her to be. I think the Predators may have re-engineered the Alien Queen in the chamber to produce quicker maturing eggs that produce quicker growing Aliens, so that the Predators could get there and have a lot of fast prey to hunt in an hour or so, instead of just sitting around all day twiddling their thumbs, reading, watching their equivalent of TV and movies, and/or playing their equivalent of cards, darts, chess, etc. while their game grows.
Then again, one simple answer could be a combination of the first two theories; varying factors can affect an outcome, which is why no two are the same.
It's also possibly that the amount of time varies from facehugger to facehugger.