It is different for every one and every pregnancy. With my first I didnt look pregnant until I was 6 almost 7 months pregnant. I had a little bump but it wasnt noticeable that much. I am now preggo with my 3rd and am 17 weeks. I am beginning to show a little but not enough to "show" i'm pregnant...just look a little chubby around the middle.
So if this is your first, it may be a while before you "look" it.
like the PP said, everyone is different. I'm pregnant with my first..I started showing at 12 weeks, i had a little bump..16 weeks, you could DEFINATELY tell im pregnant..now at 19 weeks, I look like I'm about 7 months..Causes alot of back pain=[