I've experienced it first hand. There is no reason for me to lie.
4,5-5cm on tibias maximum.
5,5-6cm on femurs maximum.
If you use externals.
There is one benefit with external femurs: your doctor can check so that your mechanical axis is aligned. That is IF he knows what he's doing.
I know the idea of LON/LATN on femurs sound very good but it's not.
You will most likely recover to a very good point but the road is devastating.
My idea with external femurs is to do 4cm and do lots of exercises and boost your body with all the proper nutrition you possibly know of and hopefully you can be out of frames in 5 months - and then you're done with it. No IMnail, no plate, no nothing.
Femurs heal faster too bc of greater blood flow.
It should be possible.
Knees will get stuck in a bad angle but it recovers with time. It took my knees a full year to recover. Today I can fully kneel, squat, anything. Even better than the average Joe.