As Ashe's ulti is described, a max of 3.5 second stun is possible. But how far must the arrow have travelled for the duration to be this long?
Active: Ashe fires a giant arrow in a straight line. If it hits an
enemy champion, it will deal magic damage and stun that champion for
1-3.5 second(s), based on the distance the arrow traveled.
Best Answer
According to the wiki, the maximum stun duration is achieved at 2800 range.