I did 8 cm on my femur and 6.2 cm on my tibias. My surgeries were roughly 3 years apart, so I did not do quadrilateral lengthening in one go, but rather femurs in 2019 (Stryde internal) and then tibias in 2023 (with LATN External TSF Frames).
Since the Stryde nail had been discontinued by the time I was deciding on my 2nd surgery and I was hesitant about using external nails, I asked Dr. Giotikas if he would consider just doing another femur lengthening for me. I wanted to be weight-bearing, so I did not seriously consider doing Precice on my tibias. The only option for weight-bearing tibias is external frames.
Giotikas flat out refused to lengthen my femurs further. He advised me that it was very unsafe to go further on the femur because no credible studies had proven that 10 cm or more on the femurs was safe. So he advised me to stick to doing tibias and that is what we did.
Every reputable surgeon I have spoken to advises "safe limits" as being 7-8 cm on femurs and/ or more than 5-6 cms on tibias. I don't personally know of any patients that went more than 8.2 cm on the femurs and/more than 7.5 cm on tibias. Keep in mind that going 7-7.5 cm on tibias carries a high risk of ballerina foot, so that needs to be proactively monitored.
So if you do femurs + tibias and hit those maximums, that would equal close to 16 cm in total length. That is certainly life changing height.
Like some of the other posters here, I have also seen diaries from patients that went all the way up to 10 cm on femurs. I think those results are atypical (maybe 2% of patients might be able to do that) and it comes with a great deal of risks.
For those kinds of diaries, I don't doubt their credibility. However, somebody who states they lengthened 20-22 cm in combined femurs and tibias would cause me serious doubt. It is important to keep in mind that anyone can write a diary on a forum and we really have no way to independently verify the claims. If someone writes a diary like that and then "disappears" once follow up questions start pouring in, I feel that could raise credibility issues.
Therefore, I would be a bit surprised if a clinic consistently advertised 20-22 cm lengthening as a typical/ achievable result for most people. It just goes against everything I have learned and experienced about CLL over the past 4 years of researching this topic extensively, interviewing surgeons, and then personally undergoing it twice.