It took over 75 lbs for people to be sure that I had lost weight. But I started off much higher than you are. Like Robin, after about 50 lbs. people starting asking if I had done something different with my hair (which I had slightly changed) and making comments that made me realize that they knew something was different but they didn't know what. Closer to 100 lbs almost everyone that I saw commented on it, so it was very obvious at that point. Upwards of 120 lbs, people that I know pretty well that have only known me as fat say they don't recognize me.
The biggest response was when I finally started buying new clothes. I kept wearing my old clothes as they were hanging off of me, sometimes with a quick dart sewn in the waistband to keep them from falling off.
When I finally had to get rid of those and get some that fit, people really noticed.