Rukia was adopted by the Kuchiki family around 48 or 49 years before Ichigo became a Shinigami. It was revealed that she immediately joined the 13th division/squad. Kaien was the lieutenant of her division at the time. It is unknown how long after this that Kaien was killed, as it has not been stated how long ago it was or how long after Rukia joined the 13th Division it was. We do know, however, than Ganju was quite a lot younger than now when it happened.