Lawjack:
I think it all depends on your starting shape. When i had started getting fatter after being in shape with decently wide shoulders it took almost 30 lbs before friends and family said anything. But if you're skinny or average shaped then I think it'd take less than 20. Also depends on how your body gains, are you more belly, legs, butt, etc.
I started with 150, so after the first 10 you could already see the difference. As it was written above - it is visible faster on a slim person.