That is a hard question to answer, there are some engineering tables out there that would put the surface flow maybe as high as 8fps.  Let’s just use 2fps or 120fpm, at 620’, that is 5 minutes.  Sometimes you use the centroid of the area so it would actually be 2.5 minutes.  Bunker, pile, feed sheds, bags would all affect the exact actual path of the water.

These assumptions are a big deal because it can cut the peak runoff by a third or half.

Are you being asked to solve the guy’s problem or give an array of choices to him?  

15” dual wall at 0.5% is 5cfs
18” is 8cfs
24” is 17
30” is 30 or whatever was shown in the pic above.


Quote the options, explain the choices, looks to me like anything less than 18” is a joke and 30” is deluxe for being regulated so the actual choice is between 18 and 24.  Or put in a waterway, might be cheaper.  Depending on what’s downstream.