Revealing mistake: When Hillard throws the basketball at Ripley, you can clearly see the white tape on Ripley's hand which was used to catch the ball. (00:23:10)
Continuity mistake: When Elgyn is going down the hallway picking up the guns, he finally comes upon a pistol. When he picks it up, there is alien slime all over it, but there is nothing else around him at all. Yet when the alien pulls him through the grating, there are gibs and assorted chunks of gore all over the place, which were not there only seconds before.
Continuity mistake: After Gen. Perez pushes the stacks of money towards Elgyn, they are right on the metallic edge of the table when the general offers a drink to Elgyn. In all of the following shots, the stacks have moved and are further in on the table, not on the metal anymore.
Chosen answer: Well firstly the queen was probably genetically engineered, like Ripley herself. A few days might be all the time they need to have a fully grown queen created. Secondly the 8 incubated victims were only the latest batch, they had been incubating people with xenomorphs for quite a time I suspect.
lionhead
With regards to the quick growth of the alien queen, it is standard for the xenomorph in nearly every film they appear (Aliens is the only exception, the only chestburster shown in the film is quickly killed by Apone) to grow to full size in around one day. Presumably the same is true for the queen.
BaconIsMyBFF