Answer: 3/4ths of an hour.
Explanation:
So we know he grows 2/3rd of a mile in half an hour, but we want to find the time to grow 1 mile at his current rate.
Let's make a ratio of miles to hours
2/3 : 1/2
4 : 3
1: 3/4
So, he can grow 1 mile in 3/4ths of an hour.