It's hard to tell these things because they change depending on/according to the situation, and you shouldn't consider my answer very seriously because I haven't done LL, as I can be getting details wrong. I'm sure some LL vets will help.
But just from the math (taking the typical distraction rate of .75mm a day with external tibias), shouldn't it take the pre-surgery + surgery time, plus around 40 days of lengthening (for 3cm), and then however long it takes for the full frame removal + consolidation phase (which I'm not sure of)? Anyone please correct me if I'm wrong.