I believe it was somewhere around a year. Some males can grow a little afterwards. I've had my Toast have a huge last minute growth spurt. He's still growing now, but not in length, in width. XD My girls seemed to stop growing much sooner than a year. I think it really varies on rat to rat when it comes how big they will get. Boys will be bigger than girls most of the time, with the exception of a few ratties I have known.