Hi JTU
Bone length is generally considered as a major factor because lengthening is a proportional matter, meaning that roughly one can lengthen a segment up to 20% of its bone. For example, if your femur is 40 cm, you may lengthen it up to 8 cm, while if someone has 30cm of femur, he/she can lengthen it 6cm at max. Of course, this is a super simplified perspective. However, it is sort of true because different sizes of bone mean that they also have different sizes of tissues like muscle. So if one who has 30cm of femur tries to lengthen femur to 8 cm, he/she will have to lengthen more proportion of tissues, as compared to one who has 40cm of femur. It is a simple math problem.
However, people are very different. Some people are more flexible than others. Some people are recovering faster than others. Generally speaking, young people are better than old ones. Female is better than male. You are young so I am sure you can recover quite well. However, flexibility is hard to achieve. Most of people have similar flexibility in my opinion unless you are ballerina or athletics. So don't really think that my flexibility is so good so I can lengthen much more than others. So you need to have a realistic goal. Once you start lengthening, you will realize how it feels.