Do you mean how long do cats grow in length, or how long before they are full grown? To the first question, they are usually about 12 to 14 inches in length. To the second, they are usually considered full grown at about 1 year.
1 Additional Answer
The growth of a cat depends on the type of cat you have. It also depends on if it is a male or female or if it is a runt. It all depends the growth varies.