After the 6 month mark following FUE, how much more hair can be expected to grow?

You are just at the stage where you are getting a good idea of what the final stage will look like. You will start to notice the most noticeable growth between 6-9 months and it takes a full year to see everything grow. 

Growth may start anytime between the first and the 5th months. Once it starts, it grows at about 1/2 inch per month. By the 8th month 80% of the hair should have grown to styling length and by the 12th month 95%.