First of all of will depend on the genetics of the fish. Also how well the owner care for it. Example, regular water change, food nutritional value, how much it eats, tank size and water params. It will vary in size. I have a runt endli and it won't grow anymore and it's only 10". It's been 3 years now and all it's other siblings are already at 18-21 inches. There are also some in my LFS that are still at 8" and they are also the same batch as the 10" endli that I have. Talk about very very very very bad genes. This is actually good news to those who can't have a big tank. But the thing is..., you will find out in 1.5 years if the endli won't grow to 12-14 inches in that year.