I would again say seconds.
I had a recent situation where I essentially arrived in the middle of an ongoing situation. What my eyes saw were two young men in gang type clothing (solid colored Dickies) moving, acting aggressively towards another individual who was seemingly trying to retreat from or avoid the situation. What my ears heard were raised and forceful voices from the two Dickies wearing gents and a low and muffled tone from the lone individual. Putting those obvious clues together, in seconds, I can make a basic assesment of a 2 on 1 altercation, possibly gang related.
I think it also comes down to mindset. Not everyone has lived through the same set of circumstances. A police officer is trained to recognize certain signals and clues. A person like me has had to live through seeing those signs and clues repeatedly and not recognizing through memorization, but learning them through the instict of survival. Others can go through varying degrees of training. Someone who has no need or purpose to recognize subtle details in a situation may not be inclined to do so.
The encounter I had lasted a minute and a half tops. I would comfortably say that within 10-15 seconds I was able to: asses the situation, establish a line of sight so I wouldn't get flanked, decide under what conditions I would present my weapon, decide who would be shot first, and decide I was just going to observe unless a specific set of conditions came into play (eg. 2 on 1 fight, someone else had a weapon, etc.). The rest of the time, I basically observed.