Soft tissue can barely stretch at all, that's why it's painful and tight after like a few cm.
You're thinking about bone growth. Tibia bone growth at that rate is ideal to avoid non union and avoid premature consolidation. You pull the bone apart too fast, now you have a big gap with no signs of connection. You pull too slow, now the bone hardens and your Lengthening device stops working and you'll need to rebreak your bone. Femur it's 1mm a day, the bone grows faster.
Pretty much you gotta go through LL to know your bone growth rate. There's not really a test or blood work to check this. For the vast majority of people, 1mm for Femurs and .75 or .67mm for Tibias is fine. Scientists have studied these numbers upon discovery of distraction osteogenesis and it's pretty uniform for most humans. Race can also plays a factor, MDOW saids Asians bone growth are slower.
Smokers also have less bone growth, so don't smoke. And weight bearing is apparently the biggest reinforcement for good bone growth, which is also why stryde is game changing. Standing vs bedridden makes all the difference.