Probably when I was 17-18 pounds down. 5 pounds was nice to see, but for all I knew, it was just water weight. 10 pounds really got my motivation in gear, but around 17 pounds down, I was under my "Whoa, nelly!" weight and went down a size in clothing. Felt really nice, and I knew I was onto something sustainable and real.
Bombe, 15 pounds isn't inconsequential. You're in Onderland, besides!

15 pounds is a fourth of your previous weight loss, and is nothing to be sneezed at. 15 pounds is a size or sometimes two worth of change. Seriously, take three 5-pound bags of sugar, maybe put them in a bag of something and try to carry them... it can give you a visual/physical representation of what you've lost. You're done a good job so far and I wish you lots of success!