This is kick-off of an Ascenso MX (Mexican second division) game between Necaxas and Mineros. This Vine begins with the opening whistle, and it ends with a 1-0 score in favor of Mineros:
Here's the full video:
That says seven seconds for the Gustavo Ramirez goal. I've seen four elsewhere. Since Vines are just a hair over six seconds at maximum, it's definitely no longer than that.
Whatever the exact time, I think we can all agree this is extremely damn fast. You've been bested, Clint Dempsey, and it's not even close.