My "common sense" rule is to treat other people as people. If you are staring at someone's chest you aren't treating them as a person, but as something to get your jollies from. I agree that someone showing a lot of skin likes attention, but even then they deserve to be treated as a person. Time frame? Keep track? It's like the pornography definition, I suppose, but I wouldn't stare at someone else's chest (male or female) while interacting with them regardless of the skin present. 

In this case, the picture was taken without her knowledge, so it seems likely she was bending down and he snapped a picture at a moment of maximum exposure. That would be lecherous without worrying about exactly how many seconds were spent "glancing".