Many UJs don't get to 15"s-18"s, but below are some examples from my own tanks. I have found growth rate many times depend on individual specimen and how big of eaters they are.
Weeksii 12"s- 15"+ : Almost 2 years: not a big eater
Teugelsi 12"s- 15"+ : 6-8 months : big eater
Ornate 12"s- 15"s+ : 10-12 monthsr: semi big eater