For me to notice it was 30 pounds. I could see it through my face, chin and neck. Something that I did that I would also recommend is taking pictures. I'm notorious for a really bad perception of myself. I lost over 100 pounds once and really couldn't tell, but that was just my mind. When I seen the pictures I can see the progression.
It took about 50 pounds for a lot of people to notice, but it probably also had to do with the fact that I had bought new form fitting pants. THAT'S when I was getting comments all over the place. A must do!
I think it was also around 50 pounds when I noticed the greater mobility. I was able to go up and down stairs again with out the weird gait or pain. THAT was a biggie for me too.