It really depends on your frame and how much you already weigh,
If your petite with only an extra 15 pounds, people may notice with five ten pounds gone.
If you are tall, and more severly overweight it may take 25 to 30 pounds, or even more.
I also find that if people see you every day they may not notice as quickly as say for example somebody that has not seen you for awhile. For people that see you everyday the change is gradual,...kinda like you dont realize how much a child has grown until you measure them from six months ago,..but when grandma comes to visit she cant believe the change!!!!!!
I have lost 67 lbs,..and really struggling to find clothes that fit in my closet, even my smaller stuff was making me look like I was wearing clown pants...so I bought some new jeans and wore them this weekend....I got lots of compliments on my weight loss, I think the clothes that finally fit helped show the changes.
I think the next 33 lbs I have to lose will show the most significant change. The first bit of weight took me from fat to less fat,..the last 33 hopefully will take me from fat to normal.