Hello budy, how are you ? I will use my measurements to make it easier to understand.
The final proportion will depend on its current proportion, biologically the femur is the longest bone of the body up to being up to 59% larger than the tibia, remember that a part of the thigh gets inside the hip then sometimes even the femur being a little the longer it appears to be the same size, but in my case I have a 50% femur ratio and 50% tibia (42cm and 42cm) it appears to be a bit short compared to tibia so a femur elongation of 8cm would be femur50cm and tibia 42cm the proportion would still be femur 54% and tibia 46% my proportion would still be normal. take the measurements and make the percentage calculation, maybe an option for you would be to divide this value in femur and tibia, if it is 3,5cm + 3,5cm in 35 days you reach 7cm total the rest is only recovery, if it is to make 7cm only in the femur it would take 70 days of stretching, outside recovery.
Another very important issue is the wingspan, I am 1.78m tall and 1.84m wingspan, and most importantly my relaxed arms down are in the middle of the length of my femur,
if I stretch it 8cm it still gets propocional below the waist, for people with arms up to the waist the final proportion with pernar longer can get strange even.
then take photos, edit the photos to get an idea of how the final proportion can be.