10% of original bone length which should be 7-8 cm max with quadrilateral (3.5 cm tibia and 4.5 cm femur). If you want double that amount, you have to re-break both segments and do the same amounts again but you'll look comical 100%.
After 8 cm total it's not only leg-to-body ratio that will start to look off but more importantly, your arm length will look t-rex level next to other men at your final post LL height.
Moreover, chances of misalignment, bad alignment or non-union increases dramatically if you go past 4 cm on tibia and 5 cm on femur in one go.
Soft tissues are another problem with overlengthening so, basically, there are tons of issues and risks invovled if you lengthen huge amounts in one go.
And, humerus lengthening, while being an option, won't look good unless your forearm:humerus ratio is off. Your forearm will look comical and your arm biomechanics will be off. If there was forearm lengthening like there is tibia lengthening, than, cosmetic arm lengthening would be a viable choice after massive LL, but, unlike with legs where you can lengthen both segments in order to preserve interlimb ratios and biomechanics, you can't do the same with arms as there's only humerus lengthening possible.
So, if you value proportions and recovery, don't go above 3 inches (3.5 cm in tibia and 4.5 cm in femurs).