I think mine became a "baby bump" at 17ish weeks. I was on my back and sucked in my stomach. What remained was a round protruding uterus, and I was like, "oh, that's the baby." I made my husband feel the tight round of my uterus too. It was quite fun.
It's different for everyone and quite frankly the baby is pushing your organs up; therefore, any showing beyond normal bloat I would consider a bump.