I'm growing out my hair color to see just how gray I am. I thinking of keeping it gray depending on how it looks. I was wondering if I cut it off as short as I can, how many inches should I leave on my head to not look totally ridiculous? Is 3 inches enough.
I have thick but fine hair.

The length and style of your hair is your choice. I think 3 inches of hair is very short. If you are hesitant about cutting your hair that short, you might want to consider letting your hair grow at least 6 inches before cutting the chemically processed hair off.