What would be the average gained height increase after LL? Or does it depend on the person?
Depends largely on the person
People that are concerned about the athletic abilities and their ability to walk properly should lengthen within safe amounts and for most thats likely in the 3-5cm range for tibias and slightly more in femurs
There are people who have done 8-10 in tibias however there is not a chance in hell these people will be able to perform at a good level athletically again. It can be done if height is your only concern but if you have other sporting interests youd better think long and hard before getting this done.