I am 9 weeks and starting to show, I posted a pic if you wanna see. I did have a little pouch already but it is getting much larger. My sis is 15 weeks with her second and was noticeably pregnant at 8 weeks, she is a very slender girl.
The above poster is right everyone is different but i think around 15wks is when most start to show. :)
I'm only 10 weeks with my 2nd, and i've popped out a lot sooner than with my first. I can't button my pants anymore, but can't quite fit into maturnity jeans. I'm wearing the tummy belt a lot of the time.
You'll probably start noticably showing just before your 4th month. My guess though. I just can tell how different this one is for me. but i dont think anyone else can tell i'm pregnant yet.