Your pants should be as long as is comfortable for you to wear. Some people like their pants to be long enough to still cover their shoes while they are sitting down. Others do not like to have their pants touching the ground while they are standing.
1 Additional Answer
Your pants should be long enough to come down to the top of the shoe and the back of the pant hem should come down just past the middle of the shoe. I am one that is very uncomfortable when I sit down and my pants ride up my leg. I like my pants long but not dragging the floor!