Clarity issue with Ashe's ultimate

Her ultimate "Enchanted Crystal Arrow" says that it flies in a straight line and deals damage upon colliding with a champion and stuns up to 3.5 seconds based on how far its travelled.

But how do we know how far 'far' would be to get the full 3.5 seconds? Does it travel over 3 seconds? or 5 seconds? or how long does it have to travel to get the full stun duration? Please include this in the next patch please. This then has to get solved by trial and error.

For reference, Jinx's ult rocket says it deals between X damage and up to X damage if allowed to travel for 1 second. So if Jinx fires a rocket and its in flight for 1 second, it does its maximum damage. Great to know. Now can we please include that for Ashe as well?