How long does it take to defrag a hard drive?

I have a windows Vista computer with a 231 GB hard drive with 21 GB free. It has not been defragmented since around three years ago. It has been in large use since then (lots of stuff that may fragment a computer such as file editing). How long should it take to defrag the hard drive?

I am only asking since it has been defragging for the past five hours and is still going.

My computer also has the option to defrag on a schedule. How often should I set it to be defragged?