You didn't state what kind of Boa constrictor, so I will assume you are asking about a Colombian boa (Bci). Bcc tend to have a somewhat slower growth rate than they do.
At 12 months age, they are usually around 3-3.5 feet on average. They grow very, very fast if provided with the right environment and diet. By 24 months of age they can be anywhere from 4.5-6 feet. I move my snakes in to their adult sized enclosures when they are around a year of age. I house all my larger boas in custom wood 4x2x1 cages.
Please seperate your snakes. They are solitary animals that do not benefit from being housed together. Many problems can arise, such as illness, stress-related problems (feeding, behaviour, etc.), and injury. This is especially a concern with maturing animals, since males will breed much earlier than females are ready. Males may breed at a year of age, while females may not be ready for several more years, and accidental breedings can be potentially fatal.
Good luck with your little friends, and welcome to the site!