At what size (inches) will my Bala Sharks out~grow my 48-inch long (55 Gallon) tank?

According to Wikipedia on line's dictionary, the Bala shark grows to a maximum length of 12 inches, or one foot in length and take a long time to get that length. However, if and when they do, your idea for getting them into a tank larger than 48 inches long is best because 12 inches is one fourth of the length of the current tank they are swimming in and that hardly seems like enough room for them to be comfortable.