Let's have a look at the evidence from each of the films:
Alien (6-12 hours)
In the film, Kane is implanted. He's brought on board and after the crew has had sufficient time to have an exciting game of "hunt-the-facehugger", conduct brief repairs to the Nostromo and get back into lightspeed (certainly no more than 12 hours tops) we see the iconic chestburster scene.
Aliens (3-6 hours?)
We don't actually see anyone being facehugged that subsequently chestburts in this film. In this deleted scene, Burke complains of being able to feel an alien inside him, suggesting that it's entering its final phase of gestation just a few hours after implantation. This conflict (with the earlier film) may be the reason they decided to remove the scene.
Alien³ (4-36 hours depending on species)
Depending on your preferred cut, we see three life-forms implanted with eggs; A dog, a cow and a human. In the theatrical cut, Ripley is brought into the facility and remains unconscious for a few hours (possibly as much as a day) during which time a dog is implanted and a chestburster emerges. In the Assembly Cut, an ox is implanted and chestbursts within no more than three to four hours, depending on how much time it took for Ripley to regain consciousness, get examined and get dressed. Ripley herself is implanted in hypersleep and gestates for approximately two to three days in realtime with the "queen embryo" emerging as she dies.
Alien Resurrection (4-8 hours?)
The entire events of the film, ignoring the off-screen action that happens before the arrival of the crew of the Betty, appears to take place over a single (24 hour?) day. During that time Purvis is facehugged, implanted and chestbursts all within that single day, potentially in as short a time as a handful of hours.
From the point that the embryo is implanted, to the point at which it emerges from the host seems to be something between 6-12 hours, depending on a number of unknown factors including, potentially, whether the embryo is a queen embryo, whether the egg is elderly and the size of the host.
Note that in the later films (AvP and Alien: Covenant) as well as the various non-canon video games the gestation period can be as little as a few minutes, something which clearly and dramatically conflicts with what we see in the earliest films.