When people say avg 5K likes per picture.. what is the duration of the pic?

It differs depending on the person making the judgement.
You can say 5K likes for 1 photo within 24 hours.
Or, you can do what most do, and add all the likes for the 10 most recent photos (as long as their not older than 30 days), and divide the total number of likes by 10. That gives you an average of the last 10 photos which is more of an accurate number.

Of course, once you start getting over 1K on 1 photo within 24 hours, that is very good. - As long as it's all natural, no fake likes.