You know what? I have no idea because I was either lied to outright or the subject wasn't broached because people thought it impolite
I couldn't tell in pictures until about 50 pounds down, and because I could still wear a lot of the same clothes, it wasn't visible to a lot of people either. When I finally went shopping and put on a few shirts in a 24 rather than the 32 I had been wearing, then I could see it. My face slimmed out around the 50 pound mark too.
It's hard for us to notice, but others do. That being said, finding an impartial observer is HARD. My one bestie would come in to my office and tell me how skinny I looked 10 pounds down. Ten pounds does not make visible difference on a 450 pound frame, but she thought she was being helpful. My other bestie said this "I can't tell. But to be fair, I didn't notice when you gained the last 150 either". Acquaintances generally won't say anything because it's an indelicate subject.
In the end too, it depends on where you lose it from. Some people's faces thin out instantly. Some just all over shrink down gradually. Myself? I lost it in my wrists ankles and jowls first. My arms and legs are a great deal smaller, but I can still wear the pants I was wearing 97 pounds ago. Everyone's mileage varies.
Go by non visual indicators like clothing size, measurements, fitting into small spaces etc.
How much weight did you loose before people started noticing?
