It depends on the individual. Generally, we consider the smaller rabbits full grown by 6 months of age. Larger breeds can take 8 months or more to reach their full size.
I am fostering a New Zealand Mix who is 4 months old. It seems likes she gets bigger and bigger every day! My own rabbits are small breeds- each one is about 3 lbs. I've read that female New Zealand's get to be 10-11 lbs. Does that sound right?