I noticed some changes right away, but that was mostly because the first place I lost weight was around my jaw.
As far as looking at your body and seeing that you look noticeably thinner though? In my experience, it's at about 10% total body weight lost. So for me, it was once I'd lost the first twenty pounds.
You have to realize that it's generally coming off just a little bit from everywhere (unless you're like me with the magically shrinking face), so it's not like you're suddenly going to lose those love handles. You're just taking it off in layers.