1. First of all it's hard to accurately measure the lengthened bone gap to the mms. Then yes, you may lose some mms but not after you take to externals off as that doesn't make any sense (your bones should be quite strong at that point), but during the consolidation phase, when your bone gap is forming and some pin bending may occur resulting in lost length. The amount of lost mms depends on the strength of the external fixator, quality of its application and the amount of weight bearing you do. You may also lose mms in the (possible) deformity correction stage.
2. The "recovery" time is individual, the usual healing index for tibia lengthening in adults is 1.5-2 months per CM lengthened. So in your case you may most likely remove the externals in 6 to 9 months. Note that you can walk unaided with the externals still on, at least with the regular circular external frames, as the bone fixation strength is godly, but that doesn't mean a thing. Removing the externals also doesn't mean you are recovered, after the removal you will need at least few months to get used to your new gait and walk normally and around an year to feel "fully recovered".
Above applies only if you don't face any serious complications during the LL process, such as nonunion, delayed consolidation, "ballerina feet" etc.