I always wondered, how long one could stare at a person without crossing any imaginary civilized boundary? More often than not, we walk across somebody who seems familiar but we immediately can’t recollect and need to visually gather more information before any memory recall happens. The time needed to gather more visual information could vary and that would mean for whole of that duration either one is staring at the person in question or is stealing glances at him or her. 
Wonder if there are acceptable time  Also we definitely want to save ourselves the embarrassment of greeting somebody who ends up being a close resemblance only of somebody we know.  Needless to say that, I am not referring to people who use this as a ploy to pursue their love interest!

limits that we draw intuitively so that the stare does not cross-over to a lecherous domain or to the domain of ‘being interested’.

But really, is there a mental limit that we have beyond which the stare we receive or give becomes uncomfortable, threatening and intrusive? Does it vary culturally? I know not! All I know is that I have shot myself in the foot few times. 
 In a friends marriage I saw couple of faces that I thought were very familiar to me. They didn’t show any indication of recognizing me, I had to look hard and try and gather as much visual details to confirm that these definitely were the ones that I know. I probably did it to an extent where the couple started showing some irritation. I had two option either stop staring, concentrate on my food and leave with the doubt permanently in my mind or to walk up to them with courage and ask if they could recognize me. For whatever reasons, I chose the latter! I walked upto them, stood before them with a smiling face and hoped that they would recognize me.  They couldn’t at all –I walked away a little embarrassed at the whole sequence of events. My friends, who saw me do this, thought that it was my habit to go upto strangers and ask if they recognized me!!