Extra point- How much clock time ?
 

NFL, during a Field goal, if the ball goes through the uprights in 2 seconds, why do they always run off at least 4 seconds ? Is there some minimum time that must be run off

They don't run off anytime on an extra point.

A) a field goal and an extra point are different things ... which are you actually asking about?
B) It takes almost two seconds to get the ball from center to holder and then kicker to ball. There is no kick that I've ever seen that comes close to sailing through the uprights just 2 seconds after the snap. Ball isn't dead until it sails beyond the end line.

I believe they have a philosophy of a certain number of seconds remaining let the clock run out on a FG attempt. That may be 5 or 6 seconds. I've heard a similar philosophy from some of my NCAA white hats when talking to the clock operator.

I believe in NCAA and NFL a FG attempt cannot take longer then 5 seconds.

There is no such rule in NCAA. There may be conference philosophy around that but I haven't seen anything in writing about it.