I don't have any way of knowing when people noticed. It wasn't until I'd lost about 50-60 pounds that anyone commented, though. Then, when I'd lost about 80 pounds, a switch seemed to flip - the comments starting coming much more frequently.
Remember that weight is something of a taboo and touchy subject. People might not comment on your weight loss when they first notice it, for fear of offending you or of broaching an inappropriate subject.
Also it's just not that easy to tell, always. Someone might think you look good, or a little different, but not be able to put their finger on why. Haircut? New glasses? Different makeup?
So don't be discouraged if people don't say anything. It doesn't mean they haven't noticed, and it definitely doesn't mean the changes you are making to yourself are making no difference. Just have a little patience.
people first commented when I had lost about 10% of my body weight. And interestingly, now that I am just about to hit another 10%, I've started getting another round of compliments. Althought I think that was in large part because I started being comfortable wearing more body conscious clothing that fits better! Not tight but stuff that fits my body rather than just covers it.
About 20-25lbs (starting from 225lbs) so about 10% of my body weight. We moved when I was about 194lbs and I've gotten some comments recently (so at about 10-15lbs lost) so just a little under 10% of my body weight again.
People outside my family didn't start noticing until very recently for me and I've lost around 20lbs. I think people don't comment more because talking about someones weight is a very touchy subject and no one wants to offend friends/family/coworkers and so on.